diff --git a/admin/inc/class.admin_ui.inc.php b/admin/inc/class.admin_ui.inc.php
index 0eeae274a0..ef1f147912 100644
--- a/admin/inc/class.admin_ui.inc.php
+++ b/admin/inc/class.admin_ui.inc.php
@@ -204,7 +204,7 @@ class admin_ui
 						$item['id'] = substr($item['extradata'], 11);
 						unset($item['extradata']);
 						$matches = null;
-						if ($item['options'] && preg_match('/(egw_openWindowCentered2?|window.open)\([^)]+,(\d+),(\d+).*(title="([^"]+)")?/', $item['options'], $matches))
+						if (!empty($item['options']) && preg_match('/(egw_openWindowCentered2?|window.open)\([^)]+,(\d+),(\d+).*(title="([^"]+)")?/', $item['options'], $matches))
 						{
 							$item['popup'] = $matches[2].'x'.$matches[3];
 							if (isset($matches[5])) $item['tooltip'] = $matches[5];
@@ -213,7 +213,7 @@ class admin_ui
 					}
 					if (empty($item['icon'])) $item['icon'] = $app.'/navbar';
 					if (empty($item['group'])) $item['group'] = $group;
-					if (empty($item['onExecute'])) $item['onExecute'] = $item['popup'] ?
+					if (empty($item['onExecute'])) $item['onExecute'] = !empty($item['popup']) ?
 						'javaScript:nm_action' : 'javaScript:app.admin.iframe_location';
 					if (!isset($item['allowOnMultiple'])) $item['allowOnMultiple'] = false;
 
@@ -297,7 +297,7 @@ class admin_ui
 					$item['id'] = substr($item['extradata'], 11);
 					unset($item['extradata']);
 					$matches = null;
-					if ($item['options'] && preg_match('/(egw_openWindowCentered2?|window.open)\([^)]+,(\d+),(\d+).*(title="([^"]+)")?/', $item['options'], $matches))
+					if (!empty($item['options']) && preg_match('/(egw_openWindowCentered2?|window.open)\([^)]+,(\d+),(\d+).*(title="([^"]+)")?/', $item['options'], $matches))
 					{
 						$item['popup'] = $matches[2].'x'.$matches[3];
 						$item['onExecute'] = 'javaScript:nm_action';
@@ -326,7 +326,7 @@ class admin_ui
 	public static function get_users(array $query, array &$rows=null)
 	{
 		$params = array(
-			'type' => (int)$query['filter'] ? (int)$query['filter'] : 'accounts',
+			'type' => (int)($query['filter'] ?? 0) ?: 'accounts',
 			'start' => $query['start'],
 			'offset' => $query['num_rows'],
 			'order' => $query['order'],
@@ -334,7 +334,7 @@ class admin_ui
 			'active' => !empty($query['active']) ? $query['active'] : false,
 		);
 		// Make sure active filter give status what it needs
-		switch($query['filter2'])
+		switch($query['filter2'] ?? '')
 		{
 			case 'disabled':
 			case 'expired':
@@ -356,12 +356,12 @@ class admin_ui
 				break;
 		}
 
-		if ($query['searchletter'])
+		if (!empty($query['searchletter']))
 		{
 			$params['query'] = $query['searchletter'];
 			$params['query_type'] = 'start';
 		}
-		elseif($query['search'])
+		elseif(!empty($query['search']))
 		{
 			$params['query'] = $query['search'];
 			$params['query_type'] = 'all';
@@ -377,7 +377,7 @@ class admin_ui
 		foreach($rows as $key => &$row)
 		{
 			// Filter by status
-			if ($need_status_filter && !static::filter_status($need_status_filter, $row))
+			if (!empty($need_status_filter) && !static::filter_status($need_status_filter, $row))
 			{
 				unset($rows[$key]);
 				$total--;
@@ -391,8 +391,8 @@ class admin_ui
 
 			if (!self::$accounts->is_active($row)) $row['status_class'] = 'adminAccountInactive';
 		}
-		// finally limit query, if status filter was used
-		if ($need_status_filter)
+		// finally, limit query, if status filter was used
+		if (!empty($need_status_filter))
 		{
 			$rows = array_values(array_slice($rows, (int)$query['start'], $query['num_rows'] ?: count($rows)));
 		}
@@ -436,9 +436,9 @@ class admin_ui
 	{
 		$groups = $GLOBALS['egw']->accounts->search(array(
 				'type'  => 'groups',
-				'query' => $query['search'],
-				'order' => $query['order'],
-				'sort'  => $query['sort'],
+				'query' => $query['search'] ?? null,
+				'order' => $query['order'] ?? null,
+				'sort'  => $query['sort'] ?? null,
 				'start' => (int)$query['start'],
 				'offset' => (int)$query['num_rows']
 			));
@@ -463,7 +463,7 @@ class admin_ui
 			$run_rights = $GLOBALS['egw']->acl->get_user_applications($group['account_id'], false, false);
 			foreach($apps as $app)
 			{
-				if((boolean)$run_rights[$app])
+				if(!empty($run_rights[$app]))
 				{
 					$group['apps'][] = $app;
 				}
@@ -537,7 +537,7 @@ class admin_ui
 					if (!empty($data['icon']))
 					{
 						$icon = Etemplate\Widget\Tree::imagePath($data['icon']);
-						if ($data['child'] || $data[Tree::CHILDREN])
+						if (!empty($data['child']) || !empty($data[Tree::CHILDREN]))
 						{
 							$data[Tree::IMAGE_FOLDER_OPEN] = $data[Tree::IMAGE_FOLDER_CLOSED] = $icon;
 						}
diff --git a/api/src/Accounts.php b/api/src/Accounts.php
index 76e80b4035..889f6c1907 100644
--- a/api/src/Accounts.php
+++ b/api/src/Accounts.php
@@ -223,7 +223,7 @@ class Accounts
 		if (!empty($param['offset']) && !isset($param['start'])) $param['start'] = 0;
 
 		// Check for lang(Group) in search - if there, we search all groups
-		$group_index = array_search(strtolower(lang('Group')), array_map('strtolower', $query = explode(' ',$param['query'])));
+		$group_index = array_search(strtolower(lang('Group')), array_map('strtolower', $query = explode(' ',$param['query'] ?? '')));
 		if($group_index !== FALSE && !(
 				in_array($param['type'], array('accounts', 'groupmembers')) || is_int($param['type'])
 		))
@@ -595,12 +595,12 @@ class Accounts
 	/**
 	 * Return formatted username for a given account_id
 	 *
-	 * @param string $account_id =null account id
-	 * @return string full name of user or "#$accountid" if user not found
+	 * @param int $account_id account id
+	 * @return string full name of user or "#$account_id" if user not found
 	 */
-	static function username($account_id=null)
+	static function username(int $account_id)
 	{
-		if ($account_id && !($account = self::cache_read((int)$account_id)))
+		if (!($account = self::cache_read($account_id)))
 		{
 			return '#'.$account_id;
 		}
diff --git a/api/src/Etemplate/Widget/Box.php b/api/src/Etemplate/Widget/Box.php
index 3a1bb39b1b..de3a379091 100644
--- a/api/src/Etemplate/Widget/Box.php
+++ b/api/src/Etemplate/Widget/Box.php
@@ -50,12 +50,12 @@ class Box extends Etemplate\Widget
 		$old_expand = $params[1];
 
 		if ($this->id && $this->type != 'groupbox') $cname = self::form_name($cname, $this->id, $params[1]);
-		if ($expand['cname'] !== $cname && trim($cname) != '')
+		if (!empty($expand['cname']) && $expand['cname'] !== $cname && trim($cname))
 		{
 			$expand['cont'] =& self::get_array(self::$request->content, $cname);
 			$expand['cname'] = $cname;
 		}
-		if ($respect_disabled && ($disabled = $this->attrs['disabled'] && self::check_disabled($this->attrs['disabled'], $expand)))
+		if ($respect_disabled && isset($this->attrs['disabled']) && self::check_disabled($this->attrs['disabled'], $expand))
 		{
 			//error_log(__METHOD__."('$method_name', ".array2string($params).', '.array2string($respect_disabled).") $this disabled='{$this->attrs['disabled']}'=".array2string($disabled).": NOT running");
 			return;
@@ -73,7 +73,7 @@ class Box extends Etemplate\Widget
 
 		// Expand children
 		$columns_disabled = null;
-		if($this->id && $this->children[0] && strpos($this->children[0]->id, '$') !== false)
+		if($this->id && isset($this->children[0]) && strpos($this->children[0]->id, '$') !== false)
 		{
 			// Need to set this so the first child can repeat
 			$expand['row'] = 0;
diff --git a/api/src/Etemplate/Widget/Date.php b/api/src/Etemplate/Widget/Date.php
index 8e7defe63c..e93a4efb9c 100644
--- a/api/src/Etemplate/Widget/Date.php
+++ b/api/src/Etemplate/Widget/Date.php
@@ -111,7 +111,7 @@ class Date extends Transformer
 		{
 			$date = Api\DateTime::server2user($value);
 		}
-		elseif ($this->attrs['data_format'] && $this->attrs['data_format'] !== 'object')
+		elseif (!empty($this->attrs['data_format']) && $this->attrs['data_format'] !== 'object')
 		{
 			$date = Api\DateTime::createFromFormat($this->attrs['data_format'], $value, Api\DateTime::$user_timezone);
 		}
diff --git a/api/src/Etemplate/Widget/Grid.php b/api/src/Etemplate/Widget/Grid.php
index 7a1ca0c84c..17433ba3e1 100644
--- a/api/src/Etemplate/Widget/Grid.php
+++ b/api/src/Etemplate/Widget/Grid.php
@@ -80,7 +80,7 @@ class Grid extends Box
 			$columns_disabled = array();
 		}
 
-		if ($respect_disabled && ($disabled = $this->attrs['disabled'] && self::check_disabled($this->attrs['disabled'], $expand)))
+		if ($respect_disabled && isset($this->attrs['disabled']) && self::check_disabled($this->attrs['disabled'], $expand))
 		{
 			//error_log(__METHOD__."('$method_name', ".array2string($params).', '.array2string($respect_disabled).") $this disabled='{$this->attrs['disabled']}'=".array2string($disabled).": NOT running");
 			$params[0] = $old_cname;
@@ -89,7 +89,7 @@ class Grid extends Box
 		}
 
 		if ($this->id && $this->type !== 'row') $cname = self::form_name($cname, $this->id, $expand);
-		if ($expand['cname'] !== $cname && $cname)
+		if (!empty($expand['cname']) && $expand['cname'] !== $cname && $cname)
 		{
 			$expand['cont'] =& self::get_array(self::$request->content, $cname);
 			$expand['cname'] = $cname;
diff --git a/api/src/Framework.php b/api/src/Framework.php
index a2fc76daa5..2a57a1f33b 100644
--- a/api/src/Framework.php
+++ b/api/src/Framework.php
@@ -495,17 +495,10 @@ abstract class Framework extends Framework\Extra
 		{
 			$lang_code = $GLOBALS['egw_info']['user']['preferences']['common']['lang'];
 		}
-		// IE specific fixes
-		if (Header\UserAgent::type() == 'msie')
-		{
-			// tell IE to use it's own mode, not old compatibility modes (set eg. via group policy for all intranet sites)
-			// has to be before any other header tags, but meta and title!!!
-			$pngfix = '<meta http-equiv="X-UA-Compatible" content="IE=edge" />'."\n";
-		}
 
 		$app = $GLOBALS['egw_info']['flags']['currentapp'];
 		$app_title = isset($GLOBALS['egw_info']['apps'][$app]) ? $GLOBALS['egw_info']['apps'][$app]['title'] : lang($app);
-		$app_header = $GLOBALS['egw_info']['flags']['app_header'] ? $GLOBALS['egw_info']['flags']['app_header'] : $app_title;
+		$app_header = $GLOBALS['egw_info']['flags']['app_header'] ?? $app_title;
 		$site_title = strip_tags($GLOBALS['egw_info']['server']['site_title'].' ['.($app_header ? $app_header : $app_title).']');
 
 		// send appheader to clientside
@@ -515,7 +508,7 @@ abstract class Framework extends Framework\Extra
 
 		$var['favicon_file'] = self::get_login_logo_or_bg_url('favicon_file', 'favicon.ico');
 
-		if ($GLOBALS['egw_info']['flags']['include_wz_tooltip'] &&
+		if (!empty($GLOBALS['egw_info']['flags']['include_wz_tooltip']) &&
 			file_exists(EGW_SERVER_ROOT.($wz_tooltip = '/phpgwapi/js/wz_tooltip/wz_tooltip.js')))
 		{
 			$include_wz_tooltip = '<script src="'.$GLOBALS['egw_info']['server']['webserver_url'].
@@ -524,7 +517,6 @@ abstract class Framework extends Framework\Extra
 		return $this->_get_css()+array(
 			'img_icon'			=> $var['favicon_file'],
 			'img_shortcut'		=> $var['favicon_file'],
-			'pngfix'        	=> $pngfix,
 			'lang_code'			=> $lang_code,
 			'charset'       	=> Translation::charset(),
 			'website_title' 	=> $site_title,
@@ -532,7 +524,7 @@ abstract class Framework extends Framework\Extra
 			'java_script'   	=> self::_get_js($extra),
 			'meta_robots'		=> $robots,
 			'dir_code'			=> lang('language_direction_rtl') != 'rtl' ? '' : ' dir="rtl"',
-			'include_wz_tooltip'=> $include_wz_tooltip,
+			'include_wz_tooltip'=> $include_wz_tooltip ?? '',
 			'webserver_url'     => $GLOBALS['egw_info']['server']['webserver_url'],
 			'darkmode'		=>  !empty(Cache::getSession('api','darkmode')) ?? $GLOBALS['egw_info']['user']['preferences']['common']['darkmode']
 		);
@@ -601,12 +593,12 @@ abstract class Framework extends Framework\Extra
 	 */
 	static function get_login_logo_or_bg_url ($type, $find_type)
 	{
-		$url = is_array($GLOBALS['egw_info']['server'][$type]) ?
+		$url = !empty($GLOBALS['egw_info']['server'][$type]) && is_array($GLOBALS['egw_info']['server'][$type]) ?
 			$GLOBALS['egw_info']['server'][$type][0] :
-			$GLOBALS['egw_info']['server'][$type];
+			$GLOBALS['egw_info']['server'][$type] ?? null;
 
-		if (substr($url, 0, 4) == 'http' ||
-			$url[0] == '/')
+		if (substr($url, 0, 4) === 'http' ||
+			!empty($url) && $url[0] === '/')
 		{
 			return $url;
 		}
@@ -800,7 +792,7 @@ abstract class Framework extends Framework\Extra
 				$index = '/index.php?menuaction='.$data['index'];
 			}
 		}
-		return self::link($index,$GLOBALS['egw_info']['flags']['params'][$app]);
+		return self::link($index, $GLOBALS['egw_info']['flags']['params'][$app] ?? '');
 	}
 
 	/**
@@ -981,7 +973,7 @@ abstract class Framework extends Framework\Extra
 			{
 				if (file_exists(EGW_SERVER_ROOT.$theme_css)) break;
 			}
-			$debug_minify = $GLOBALS['egw_info']['server']['debug_minify'] === 'True';
+			$debug_minify = !empty($GLOBALS['egw_info']['server']['debug_minify']) && $GLOBALS['egw_info']['server']['debug_minify'] === 'True';
 			if (!$debug_minify && file_exists(EGW_SERVER_ROOT.($theme_min_css = str_replace('.css', '.min.css', $theme_css))))
 			{
 				//error_log(__METHOD__."() Framework\CssIncludes::get()=".array2string(Framework\CssIncludes::get()));
@@ -1095,8 +1087,7 @@ abstract class Framework extends Framework\Extra
 		if(@isset($_GET['menuaction']))
 		{
 			list(, $class) = explode('.',$_GET['menuaction']);
-			if(is_array($GLOBALS[$class]->public_functions) &&
-				$GLOBALS[$class]->public_functions['java_script'])
+			if (!empty($GLOBALS[$class]->public_functions['java_script']))
 			{
 				$java_script .= $GLOBALS[$class]->java_script();
 			}
@@ -1524,8 +1515,8 @@ abstract class Framework extends Framework\Extra
 		foreach(Framework\CssIncludes::get() as $path)
 		{
 			unset($query);
-			list($path,$query) = explode('?',$path,2);
-			$path .= '?'. ($query ? $query : filemtime(EGW_SERVER_ROOT.$path));
+			list($path,$query) = explode('?', $path,2)+[null,null];
+			$path .= '?'. ($query ?? filemtime(EGW_SERVER_ROOT.$path));
 			$response->includeCSS($GLOBALS['egw_info']['server']['webserver_url'].$path);
 		}
 
diff --git a/api/src/Hooks.php b/api/src/Hooks.php
index 889c12a956..5ac92161cd 100644
--- a/api/src/Hooks.php
+++ b/api/src/Hooks.php
@@ -63,8 +63,8 @@ class Hooks
 		$location = is_array($args) ? (isset($args['hook_location']) ? $args['hook_location'] : $args['location']) : $args;
 
 		if (!isset(self::$locations)) self::read();
+		if (empty(self::$locations[$location])) return [];	// not a single app implements that hook
 		$hooks = self::$locations[$location];
-		if (!isset($hooks) || empty($hooks)) return array();	// not a single app implements that hook
 
 		$apps = array_keys($hooks);
 		if (!$no_permission_check)
@@ -115,7 +115,7 @@ class Hooks
 		}
 
 		$ret = array();
-		foreach((array)self::$locations[$location][$appname] as $hook)
+		foreach(self::$locations[$location][$appname] ?? [] as $hook)
 		{
 			try {
 				// old style file hook
@@ -130,7 +130,7 @@ class Hooks
 					return true;
 				}
 
-				list($class, $method) = explode('::', $hook);
+				list($class, $method) = explode('::', $hook)+[null,null];
 
 				// static method of an autoloadable class
 				if (isset($method) && class_exists($class))
diff --git a/api/src/Link.php b/api/src/Link.php
index 46c310e169..68298cfb91 100644
--- a/api/src/Link.php
+++ b/api/src/Link.php
@@ -762,7 +762,7 @@ class Link extends Link\Storage
 			if ($must_support && !isset($reg[$must_support])) continue;
 
 			list($app) = explode('-', $type);
-			if ($GLOBALS['egw_info']['user']['apps'][$app])
+			if (!empty($GLOBALS['egw_info']['user']['apps'][$app]))
 			{
 				$apps[$type] = lang(self::get_registry($type, 'name'));
 			}
@@ -1132,7 +1132,7 @@ class Link extends Link\Storage
 	 */
 	static function get_registry($app, $name, $url_id=false)
 	{
-		$reg = self::$app_register[$app];
+		$reg = self::$app_register[$app] ?? null;
 
 		if (!isset($reg)) return false;
 
@@ -1181,7 +1181,7 @@ class Link extends Link\Storage
 			return $str;
 		}
 
-		return isset($reg) ? $reg[$name] : false;
+		return $reg[$name] ?? false;
 	}
 
 	/**
diff --git a/api/src/Mail.php b/api/src/Mail.php
index 26760e663a..3da91ce9ea 100644
--- a/api/src/Mail.php
+++ b/api/src/Mail.php
@@ -2950,7 +2950,7 @@ class Mail
 			// Trigger examination of namespace to retrieve
 			// folders located in other and shared; needed only for some servers
 			if (is_null(self::$mailConfig)) self::$mailConfig = Config::read('mail');
-			if (self::$mailConfig['examineNamespace'])
+			if (!empty(self::$mailConfig['examineNamespace']))
 			{
 				$prefixes=array();
 				if (is_array($nameSpace))
@@ -3014,7 +3014,7 @@ class Mail
 					$subFolders = $this->icServer->getMailboxes($node['MAILBOX'].$node['delimiter'], $_search, true);
 				}
 
-				if (is_array($mainFolder['INBOX']))
+				if (isset($mainFolder['INBOX']) && is_array($mainFolder['INBOX']))
 				{
 					// Array container of auto folders
 					$aFolders = array();
@@ -3180,7 +3180,7 @@ class Mail
 				{
 					foreach(array('others','shared') as $type)
 					{
-						if ($nameSpace[$type]['prefix_present']&&$nameSpace[$type]['prefix'])
+						if (!empty($nameSpace[$type]['prefix_present']) && !empty($nameSpace[$type]['prefix']))
 						{
 							if (substr($k,0,strlen($nameSpace[$type]['prefix']))==$nameSpace[$type]['prefix']||
 								substr($k,0,strlen($nameSpace[$type]['prefix'])-strlen($nameSpace[$type]['delimiter']))==substr($nameSpace[$type]['prefix'],0,strlen($nameSpace[$type]['delimiter'])*-1)) {
@@ -3211,7 +3211,7 @@ class Mail
 			}
 			//error_log(__METHOD__.__LINE__.array2string($autoFolderObjects));
 			if (!$isGoogleMail) {
-				$folders = array_merge($inboxFolderObject,$autoFolderObjects,(array)$inboxSubFolderObjects,(array)$folders,(array)$typeFolderObject['others'],(array)$typeFolderObject['shared']);
+				$folders = array_merge($inboxFolderObject,$autoFolderObjects,(array)$inboxSubFolderObjects,(array)$folders,(array)$typeFolderObject['others'] ?? [],(array)$typeFolderObject['shared'] ?? []);
 			} else {
 				// avoid calling sortByAutoFolder as it is not regarding subfolders
 				$gAutoFolderObjectsTmp = $googleAutoFolderObjects;
@@ -3910,7 +3910,7 @@ class Mail
 	{
 		//error_log(__METHOD__.' ('.__LINE__.') '.'->'.array2string($_messageUID).','.array2string($_folder).', '.$_forceDeleteMethod);
 		$oldMailbox = '';
-		if (is_null($_folder) || empty($_folder)) $_folder = $this->sessionData['mailbox'];
+		if (empty($_folder) && !empty($this->sessionData['mailbox'])) $_folder = $this->sessionData['mailbox'];
 		if (empty($_messageUID))
 		{
 			if (self::$debug) error_log(__METHOD__." no messages Message(s): ".implode(',',$_messageUID));
diff --git a/api/src/Mail/Account.php b/api/src/Mail/Account.php
index 1bad097340..57d21be166 100644
--- a/api/src/Mail/Account.php
+++ b/api/src/Mail/Account.php
@@ -313,7 +313,7 @@ class Account implements \ArrayAccess
 		try {
 			if ($this->acc_imap_type != __NAMESPACE__.'\\Imap' &&
 				// do NOT query IMAP server, if we are in forward-only delivery-mode, imap will NOT answer, as switched off for that account!
-				$this->params['deliveryMode'] != Smtp::FORWARD_ONLY && $need_quota &&
+				($this->params['deliveryMode'] ?? null) != Smtp::FORWARD_ONLY && $need_quota &&
 				$this->imapServer($this->user) && is_a($this->imapServer, __NAMESPACE__.'\\Imap') &&
 				($data = $this->imapServer->getUserData($GLOBALS['egw']->accounts->id2name($this->user))))
 			{
@@ -335,7 +335,7 @@ class Account implements \ArrayAccess
 		}
 		$this->params += array_fill_keys(self::$user_data, null);	// make sure all keys exist now
 
-		return (array)$data + (array)$smtp_data;
+		return ($data ?? []) + ($smtp_data ?? []);
 	}
 
 	/**
@@ -454,10 +454,10 @@ class Account implements \ArrayAccess
 					$this->smtpServer->host = 'tls://'.$this->smtpServer->host;
 			}
 			$this->smtpServer->smtpAuth = !empty($this->params['acc_smtp_username']);
-			$this->smtpServer->username = $this->params['acc_smtp_username'];
-			$this->smtpServer->password = $this->params['acc_smtp_password'];
+			$this->smtpServer->username = $this->params['acc_smtp_username'] ?? null;
+			$this->smtpServer->password = $this->params['acc_smtp_password'] ?? null;
 			$this->smtpServer->defaultDomain = $this->params['acc_domain'];
-			$this->smtpServer->loginType = $this->params['acc_imap_login_type'];
+			$this->smtpServer->loginType = $this->params['acc_imap_login_type'] ?? null;
 		}
 		return $this->smtpServer;
 	}
@@ -676,7 +676,7 @@ class Account implements \ArrayAccess
 		$to_replace = array();
 		foreach($fields as $name)
 		{
-			if (strpos($identity[$name], '{{') !== false || strpos($identity[$name], '$$') !== false)
+			if (!empty($identity[$name]) && (strpos($identity[$name], '{{') !== false || strpos($identity[$name], '$$') !== false))
 			{
 				$to_replace[$name] = $identity[$name];
 			}
@@ -837,7 +837,7 @@ class Account implements \ArrayAccess
 			// let getUserData "know" if we are interested in quota (requiring IMAP login) or not
 			$this->getUserData(substr($name, 0, 5) === 'quota');
 		}
-		return $this->params[$name];
+		return $this->params[$name] ?? null;
 	}
 
 	/**
@@ -1578,7 +1578,7 @@ class Account implements \ArrayAccess
 				'ident_realname' => $account['ident_realname'],
 				'ident_org' => $account['ident_org'],
 				'ident_email' => $account['ident_email'],
-				'acc_name' => $account['acc_name'],
+				'acc_name' => $account['acc_name'] ?? null,
 				'acc_imap_username' => $account['acc_imap_username'],
 				'acc_imap_logintype' => $account['acc_imap_logintype'],
 				'acc_domain' => $account['acc_domain'],
@@ -1605,7 +1605,7 @@ class Account implements \ArrayAccess
 					}
 				}
 				// fill an empty ident_realname or ident_email of current user with data from user account
-				if ($replace_placeholders && (!isset($account_id) || $account_id == $GLOBALS['egw_info']['user']['acount_id']))
+				if ($replace_placeholders && (!isset($account_id) || $account_id == $GLOBALS['egw_info']['user']['account_id']))
 				{
 					if (empty($account['ident_realname'])) $account['ident_realname'] = $GLOBALS['egw_info']['user']['account_fullname'];
 					if (empty($account['ident_email'])) $account['ident_email'] = $GLOBALS['egw_info']['user']['account_email'];
diff --git a/api/src/Mail/Notifications.php b/api/src/Mail/Notifications.php
index a9f2d1d521..d1a1d02301 100644
--- a/api/src/Mail/Notifications.php
+++ b/api/src/Mail/Notifications.php
@@ -87,7 +87,7 @@ class Notifications
 				$account_specific = $account_id;
 			}
 		}
-		$folders = (array)self::$cache[$acc_id][$account_specific];
+		$folders = self::$cache[$acc_id][$account_specific] ?? [];
 		if (!$return_empty_marker && $folders == array(null)) $folders = array();
 		$result = array(
 			'notify_folders' => $folders,
diff --git a/api/src/Session.php b/api/src/Session.php
index 509f9409aa..29d6692975 100644
--- a/api/src/Session.php
+++ b/api/src/Session.php
@@ -1538,21 +1538,21 @@ class Session
 		}
 
 		// check if the url already contains a query and ensure that vars is an array and all strings are in extravars
-		if (strpos($ret_url=$url, '?') !== false) list($ret_url,$othervars) = explode('?', $url, 2);
+		if (strpos($ret_url=$url, '?') !== false) list($ret_url,$othervars) = explode('?', $url, 2)+[null,null];
 		if ($extravars && is_array($extravars))
 		{
 			$vars += $extravars;
 			$extravars = $othervars;
 		}
-		else
+		elseif (!empty($othervars))
 		{
-			if ($othervars) $extravars .= ($extravars?'&':'').$othervars;
+			$extravars .= ($extravars ? '&' : '') . $othervars;
 		}
 
 		// parse extravars string into the vars array
-		if ($extravars)
+		if (!empty($extravars))
 		{
-			foreach(explode('&',$extravars) as $expr)
+			foreach(explode('&', $extravars) as $expr)
 			{
 				list($var,$val) = explode('=', $expr,2);
 				if (strpos($val,'%26') != false) $val = str_replace('%26','&',$val);	// make sure to not double encode &
diff --git a/api/src/Storage/Merge.php b/api/src/Storage/Merge.php
index 51c2240820..5ae3b7575e 100644
--- a/api/src/Storage/Merge.php
+++ b/api/src/Storage/Merge.php
@@ -48,7 +48,12 @@ abstract class Merge
 	/**
 	 * Fields that are to be treated as datetimes, when merged into spreadsheets
 	 */
-	var $date_fields = array();
+	var $date_fields = [];
+
+	/**
+	 * Fields that are numeric, for special numeric handling
+	 */
+	protected $numeric_fields = [];
 
 	/**
 	 * Mimetype of document processed by merge
@@ -77,10 +82,10 @@ abstract class Merge
 	 */
 	public $export_limit;
 
-
 	public $public_functions = array(
 		"merge_entries"		=> true
 	);
+
 	/**
 	 * Configuration for HTML Tidy to clean up any HTML content that is kept
 	 */
@@ -237,7 +242,7 @@ abstract class Merge
 		$replacements = array();
 		foreach(array_keys($this->contacts->contact_fields) as $name)
 		{
-			$value = $contact[$name];
+			$value = $contact[$name] ?? null;
 			switch($name)
 			{
 				case 'created': case 'modified':
@@ -333,7 +338,9 @@ abstract class Merge
 				$cats[$cat_id] = array();
 			}
 		}
-		foreach($cats as $main => $cat) {
+		$replacements['$$'.($prefix ? $prefix.'/':'').'categories$$'] = '';
+		foreach($cats as $main => $cat)
+		{
 			$replacements['$$'.($prefix ? $prefix.'/':'').'categories$$'] .= $GLOBALS['egw']->categories->id2name($main,'name')
 				. (count($cat) > 0 ? ': ' : '') . implode(', ', $cats[$main]) . "\n";
 		}
@@ -818,7 +825,7 @@ abstract class Merge
 			$content = preg_replace(array_keys($fix),array_values($fix),$content);
 			//die("<pre>".htmlspecialchars($content)."</pre>\n");
 		}
-		list($contentstart,$contentrepeat,$contentend) = preg_split('/\$\$pagerepeat\$\$/',$content,-1, PREG_SPLIT_NO_EMPTY);  //get differt parts of document, seperatet by Pagerepeat
+		list($contentstart,$contentrepeat,$contentend) = preg_split('/\$\$pagerepeat\$\$/',$content,-1, PREG_SPLIT_NO_EMPTY)+[null,null,null];  //get differt parts of document, seperatet by Pagerepeat
 		if ($mimetype == 'text/plain' && $ids && count($ids) > 1)
 		{
 			// textdocuments are simple, they do not hold start and end, but they may have content before and after the $$pagerepeat$$ tag
@@ -862,11 +869,11 @@ abstract class Merge
 			$contentstart .= '<w:body>';
 			$contentend = '</w:body></w:document>';
 		}
-		list($Labelstart,$Labelrepeat,$Labeltend) = preg_split('/\$\$label\$\$/',$contentrepeat,-1, PREG_SPLIT_NO_EMPTY);  //get the Lable content
+		list($Labelstart,$Labelrepeat,$Labeltend) = preg_split('/\$\$label\$\$/',$contentrepeat,-1, PREG_SPLIT_NO_EMPTY)+[null,null,null];  //get the label content
 		preg_match_all('/\$\$labelplacement\$\$/',$contentrepeat,$countlables, PREG_SPLIT_NO_EMPTY);
 		$countlables = count($countlables[0]);
 		preg_replace('/\$\$labelplacement\$\$/','',$Labelrepeat,1);
-		if ($countlables > 1) $lableprint = true;
+		$lableprint = $countlables > 1;
 		if (count($ids) > 1 && !$contentrepeat)
 		{
 			$err = lang('for more than one contact in a document use the tag pagerepeat!');
@@ -1111,7 +1118,7 @@ abstract class Merge
 		{
 			foreach($this->date_fields as $field)
 			{
-				if(($value = $replacements['$$'.$field.'$$']))
+				if(($value = $replacements['$$'.$field.'$$'] ?? null))
 				{
 					$time = Api\DateTime::createFromFormat('+'.Api\DateTime::$user_dateformat.' '.Api\DateTime::$user_timeformat.'*', $value);
 					$replacements['$$'.$field.'/date$$'] = $time ? $time->format(Api\DateTime::$user_dateformat)  : '';
@@ -1234,7 +1241,8 @@ abstract class Merge
 			// Look for numbers, set their value if needed
 			if(property_exists($this,'numeric_fields') || count($names))
 			{
-				foreach((array)$this->numeric_fields as $fieldname) {
+				foreach($this->numeric_fields as $fieldname)
+				{
 					$names[] = preg_quote($fieldname,'/');
 				}
 				$this->format_spreadsheet_numbers($content, $names, $mimetype.$mso_application_progid);
@@ -1327,7 +1335,8 @@ abstract class Merge
 	 */
 	protected function format_spreadsheet_numbers(&$content, $names, $mimetype)
 	{
-		foreach((array)$this->numeric_fields as $fieldname) {
+		foreach($this->numeric_fields as $fieldname)
+		{
 			$names[] = preg_quote($fieldname,'/');
 		}
 		switch($mimetype)
@@ -1351,7 +1360,7 @@ abstract class Merge
 
 				break;
 		}
-		if($format && $names)
+		if (!empty($format) && $names)
 		{
 			// Dealing with backtrack limit per AmigoJack 10-Jul-2010 comment on php.net preg-replace docs
 			do {
diff --git a/api/src/Vfs.php b/api/src/Vfs.php
index 9829804a81..44a29de9c0 100644
--- a/api/src/Vfs.php
+++ b/api/src/Vfs.php
@@ -1255,7 +1255,7 @@ class Vfs extends Vfs\Base
 			array_push($parts,'');	// scheme://host is wrong (no path), has to be scheme://host/
 		}
 		//error_log(__METHOD__."($url)=".implode('/',$parts).($query ? '?'.$query : ''));
-		return implode('/',$parts).($query ? '?'.$query : '');
+		return implode('/',$parts).(!empty($query) ? '?'.$query : '');
 	}
 
 	/**
diff --git a/calendar/inc/class.calendar_hooks.inc.php b/calendar/inc/class.calendar_hooks.inc.php
index 612789dccb..d20893b50d 100644
--- a/calendar/inc/class.calendar_hooks.inc.php
+++ b/calendar/inc/class.calendar_hooks.inc.php
@@ -901,7 +901,7 @@ END:VALARM';
 	{
 		Api\Translation::add_app('calendar');
 		// do not set actions for alarm type
-		if ($params['data']['type'] == 6)
+		if (isset($params['data']['type']) && $params['data']['type'] == 6)
 		{
 			if (!empty($params['data']['videoconference'])
 				&& !self::isVideoconferenceDisabled())
@@ -917,6 +917,8 @@ END:VALARM';
 			}
 			return array();
 		}
+		if (!isset($params['data']['event_id'])) $params['data']['event_id'] = '';
+		if (!isset($params['data']['user_id'])) $params['data']['user_id'] = '';
 		return array(
 			array(
 				'id' => 'A',
diff --git a/mail/inc/class.mail_tree.inc.php b/mail/inc/class.mail_tree.inc.php
index 9c330849d2..22350398b9 100644
--- a/mail/inc/class.mail_tree.inc.php
+++ b/mail/inc/class.mail_tree.inc.php
@@ -145,7 +145,7 @@ class mail_tree
 	 */
 	private static function isAccountNode ($_node)
 	{
-		list(,$leaf) = explode(self::DELIMITER, $_node);
+		list(,$leaf) = explode(self::DELIMITER, $_node)+[null,null];
 		if ($leaf || $_node == null) return false;
 		return true;
 	}
@@ -404,7 +404,7 @@ class mail_tree
 			}
 			$parents[] = $component;
 		}
-		if ($data['folderarray']['delimiter'] && $data['folderarray']['MAILBOX'])
+		if (!empty($data['folderarray']['delimiter']) && !empty($data['folderarray']['MAILBOX']))
 		{
 			$path = explode($data['folderarray']['delimiter'], $data['folderarray']['MAILBOX']);
 			$folderName = array_pop($path);
@@ -428,7 +428,7 @@ class mail_tree
 					$data[Tree::IMAGE_FOLDER_OPEN] =
 					$data [Tree::IMAGE_FOLDER_CLOSED] = basename(Api\Image::find('mail', 'dhtmlxtree/'."MailFolder".$key));
 			}
-			elseif(stripos(array2string($data['folderarray']['attributes']),'\noselect')!== false)
+			elseif(!empty($data['folderarray']['attributes']) && stripos(array2string($data['folderarray']['attributes']),'\noselect') !== false)
 			{
 				$data[Tree::IMAGE_LEAF] = self::$leafImages['folderNoSelectClosed'];
 				$data[Tree::IMAGE_FOLDER_OPEN] = self::$leafImages['folderNoSelectOpen'];
@@ -444,7 +444,7 @@ class mail_tree
 			}
 
 			// Contains unseen mails for the folder
-			$unseen = $data['folderarray']['counter']['UNSEEN'];
+			$unseen = $data['folderarray']['counter']['UNSEEN'] ?? 0;
 
 			// if there's unseen mails then change the label and style
 			// accordingly to indicate useen mails
@@ -477,7 +477,7 @@ class mail_tree
 		foreach(Mail\Account::search(true, false) as $acc_id => $accObj)
 		{
 			if (!$accObj->is_imap()|| $_profileID && $acc_id != $_profileID) continue;
-			$identity = self::getIdentityName(Mail\Account::identity_name($accObj,true,$GLOBALS['egw_info']['user']['acount_id'], true));
+			$identity = self::getIdentityName(Mail\Account::identity_name($accObj,true, $GLOBALS['egw_info']['user']['account_id'], true));
 			// Open top level folders for active account
 			$openActiveAccount = $GLOBALS['egw_info']['user']['preferences']['mail']['ActiveProfileID'] == $acc_id?1:0;