diff --git a/api/src/Accounts/Sql.php b/api/src/Accounts/Sql.php
index 24efe40cec..25e999439a 100644
--- a/api/src/Accounts/Sql.php
+++ b/api/src/Accounts/Sql.php
@@ -436,12 +436,12 @@ class Sql
 		);
 
 		// fetch order of account_fullname from Api\Accounts::format_username
-		if (strpos($param['order'],'account_fullname') !== false)
+		if (strpos($param['order'] ?? '','account_fullname') !== false)
 		{
 			$param['order'] = str_replace('account_fullname', preg_replace('/[ ,]+/',',',str_replace(array('[',']'),'',
 				Api\Accounts::format_username('account_lid','account_firstname','account_lastname'))), $param['order']);
 		}
-		$order = str_replace(array_keys($order2contact),array_values($order2contact),$param['order']);
+		$order = str_replace(array_keys($order2contact),array_values($order2contact),$param['order'] ?? '');
 
 		// allways add 'account_lid'
 		if (strpos($order, 'account_lid') === false)
diff --git a/api/src/DateTime.php b/api/src/DateTime.php
index d0aecbd213..852977f209 100644
--- a/api/src/DateTime.php
+++ b/api/src/DateTime.php
@@ -189,11 +189,11 @@ class DateTime extends \DateTime
 	 *
 	 * @param DateInterval|string $interval eg. '1 day', '-2 weeks'
 	 */
-	public function add($interval)
+	public function add($interval) : \DateTime
 	{
 		if (is_string($interval)) $interval = DateInterval::createFromDateString($interval);
 
-		parent::add($interval);
+		return parent::add($interval);
 	}
 
 	/**
@@ -344,6 +344,7 @@ class DateTime extends \DateTime
 	 * 		true = date only, false = time only as in user prefs, '' = date+time as in user prefs, 'utc'=regular timestamp in UTC
 	 * @return int|string|array|datetime see $type
 	 */
+	#[\ReturnTypeWillChange]
 	public function format($type='')
 	{
 		switch((string)$type)
@@ -758,4 +759,4 @@ class DateTime extends \DateTime
 		return $ret_user_tzs;
 	}
 }
-DateTime::init();
+DateTime::init();
\ No newline at end of file
diff --git a/api/src/Db.php b/api/src/Db.php
index a6ece4875f..aa90ff2dfb 100644
--- a/api/src/Db.php
+++ b/api/src/Db.php
@@ -1603,7 +1603,7 @@ class Db
 			$value = DateTime::user2server($value,'string');
 		}
 		// truncate to long strings for varchar(X) columns as PostgreSQL and newer MySQL/MariaDB given an error otherwise
-		if (!is_null($length) && mb_strlen($value) > $length)
+		if (isset($length) && isset($value) && mb_strlen($value) > $length)
 		{
 			$value = mb_substr($value, 0, $length);
 		}
@@ -1615,7 +1615,7 @@ class Db
 		// (MariaDB 10.1 does the replacement automatic, 10.0 cuts everything off behind and MySQL gives an error)
 		// (MariaDB 10.3 gives an error too: Incorrect string value: '\xF0\x9F\x98\x8A\x0AW...')
 		// Changing charset to utf8mb4 requires schema update, shortening of some indexes and probably have negative impact on performace!
-		if (substr($this->Type, 0, 5) == 'mysql')
+		if (isset($value) && substr($this->Type, 0, 5) === 'mysql')
 		{
 			$value = preg_replace('/[\x{10000}-\x{10FFFF}]/u', "\xEF\xBF\xBD", $value);
 		}
diff --git a/api/src/Header/UserAgent.php b/api/src/Header/UserAgent.php
index 81f2d18662..4fa2855b00 100644
--- a/api/src/Header/UserAgent.php
+++ b/api/src/Header/UserAgent.php
@@ -98,9 +98,9 @@ class UserAgent
 	{
 		// should be Ok for all HTML 4 compatible browsers
 		$parts = $all_parts = null;
-		if(!preg_match('/compatible; ([a-z]+)[\/ ]+([0-9.]+)/i',$_SERVER['HTTP_USER_AGENT'],$parts))
+		if(!preg_match('/compatible; ([a-z]+)[\/ ]+([0-9.]+)/i',$_SERVER['HTTP_USER_AGENT'] ?? '',$parts))
 		{
-			preg_match_all('/([a-z]+)\/([0-9.]+)/i',$_SERVER['HTTP_USER_AGENT'],$all_parts,PREG_SET_ORDER);
+			preg_match_all('/([a-z]+)\/([0-9.]+)/i',$_SERVER['HTTP_USER_AGENT'] ?? '',$all_parts,PREG_SET_ORDER);
 			$parts = array_pop($all_parts);
 			foreach($all_parts as $p)
 			{
@@ -112,7 +112,7 @@ class UserAgent
 			}
 		}
 		list(,self::$user_agent,self::$ua_version) = $parts;
-		if ((self::$user_agent = strtolower(self::$user_agent)) == 'version') self::$user_agent = 'opera';
+		if ((self::$user_agent = strtolower(self::$user_agent ?? '')) == 'version') self::$user_agent = 'opera';
 		// IE no longer reports MSIE, but "Trident/7.0; rv:11.0"
 		if (self::$user_agent=='trident')
 		{
@@ -133,9 +133,9 @@ class UserAgent
 			self::$ua_version = '12.0';
 		}
 		self::$ua_mobile = preg_match('/(iPhone|iPod|iPad|Android|SymbianOS|Blackberry|Kindle|Opera Mobi|Windows Phone)/i',
-			$_SERVER['HTTP_USER_AGENT'], $matches) ? strtolower($matches[1]) : null;
+			$_SERVER['HTTP_USER_AGENT'] ?? '', $matches) ? strtolower($matches[1]) : null;
 
 		//error_log("HTTP_USER_AGENT='$_SERVER[HTTP_USER_AGENT]', UserAgent: '".self::$user_agent."', Version: '".self::$ua_version."', isMobile=".array2string(self::$ua_mobile));
 	}
 }
-UserAgent::_init_static();
+UserAgent::_init_static();
\ No newline at end of file
diff --git a/api/src/Mail/Account.php b/api/src/Mail/Account.php
index 6e8a7d1b47..b77f76f6a8 100644
--- a/api/src/Mail/Account.php
+++ b/api/src/Mail/Account.php
@@ -1245,7 +1245,7 @@ class Account implements \ArrayAccess
 		$backup = array();
 		foreach(array('acc_smtp_type', 'acc_imap_type') as $attr)
 		{
-			if (substr($data[$attr], 0, $ns_len) == __NAMESPACE__.'\\')
+			if (substr($data[$attr] ?? '', 0, $ns_len) == __NAMESPACE__.'\\')
 			{
 				$backup[$attr] = $data[$attr];
 				$data[$attr] = substr($data[$attr], $ns_len);
diff --git a/api/src/Session.php b/api/src/Session.php
index 1e9b6f04d3..1a038b8ff7 100644
--- a/api/src/Session.php
+++ b/api/src/Session.php
@@ -1582,7 +1582,7 @@ class Session
 				}
 				else
 				{
-					$query[] = $key.'='.urlencode($value);
+					$query[] = $key.'='.urlencode($value ?? '');
 				}
 			}
 			$ret_url .= '?' . implode('&',$query);
diff --git a/api/src/Storage/Merge.php b/api/src/Storage/Merge.php
index 57906cb9bf..ff69c378d3 100644
--- a/api/src/Storage/Merge.php
+++ b/api/src/Storage/Merge.php
@@ -2062,7 +2062,7 @@ abstract class Merge
 	 * @return string with error-message on error
 	 * @throws Api\Exception
 	 */
-	public function merge_file($document, $ids, &$name = '', $dirs = '', &$header)
+	public function merge_file($document, $ids, &$name = '', $dirs = '', &$header = null)
 	{
 		//error_log(__METHOD__."('$document', ".array2string($ids).", '$name', dirs='$dirs') ->".function_backtrace());
 		if(($error = $this->check_document($document, $dirs)))
diff --git a/importexport/inc/class.importexport_definitions_bo.inc.php b/importexport/inc/class.importexport_definitions_bo.inc.php
index 62bf782460..aa3885172b 100644
--- a/importexport/inc/class.importexport_definitions_bo.inc.php
+++ b/importexport/inc/class.importexport_definitions_bo.inc.php
@@ -250,7 +250,7 @@ class importexport_definitions_bo {
 			$definition = new importexport_definition( $definition_data['name'] );
 
 			// Only update if the imported is newer
-			if(strtotime($definition->modified) < strtotime($definition_data['modified']) || $definition->modified == 0)
+			if(!$definition->modified || strtotime($definition->modified) < strtotime($definition_data['modified']))
 			{
 				$definition_id = $definition->get_identifier() ? $definition->get_identifier() : NULL;
 
@@ -358,5 +358,4 @@ class importexport_definitions_bo {
 	}
 
 
-}
-
+}
\ No newline at end of file
diff --git a/importexport/inc/class.importexport_helper_functions.inc.php b/importexport/inc/class.importexport_helper_functions.inc.php
index b032a6c262..f6e019b06c 100755
--- a/importexport/inc/class.importexport_helper_functions.inc.php
+++ b/importexport/inc/class.importexport_helper_functions.inc.php
@@ -127,7 +127,7 @@ class importexport_helper_functions {
 	 * @return mixed comma seperated list or array with ids
 	 */
 	public static function account_name2id( &$_account_lids ) {
-		$account_lids = is_array( $_account_lids ) ? $_account_lids : explode( ',', $_account_lids );
+		$account_lids = is_array( $_account_lids ) ? $_account_lids : explode( ',', $_account_lids ?? '' );
 		$skip = false;
 		foreach ( $account_lids as $key => $account_lid ) {
 			if($skip) {
@@ -777,4 +777,4 @@ class importexport_helper_functions {
 		}
 		return null;
 	}
-} // end of importexport_helper_functions
+} // end of importexport_helper_functions
\ No newline at end of file
diff --git a/setup/inc/class.setup.inc.php b/setup/inc/class.setup.inc.php
index c4d0b06b28..6b9b4077a6 100644
--- a/setup/inc/class.setup.inc.php
+++ b/setup/inc/class.setup.inc.php
@@ -831,8 +831,8 @@ class setup
 			echo'<br>Input values: '
 				. 'A="'.$a.'", B="'.$b.'"';
 		}
-		$newa = str_replace('pre','.',$a);
-		$newb = str_replace('pre','.',$b);
+		$newa = str_replace('pre','.', $a ?? '');
+		$newb = str_replace('pre','.', $b ?? '');
 		$testa = explode('.',$newa);
 		if(@$testa[1] == '')
 		{
diff --git a/setup/inc/class.setup_cmd_config.inc.php b/setup/inc/class.setup_cmd_config.inc.php
index 890accaa39..8647813e1f 100644
--- a/setup/inc/class.setup_cmd_config.inc.php
+++ b/setup/inc/class.setup_cmd_config.inc.php
@@ -348,10 +348,10 @@ class setup_cmd_config extends setup_cmd
 	 */
 	function _save_mail_account(array $data)
 	{
-		// convert ssl textual values to nummerical ones used in Api\Mail\Account
+		// convert ssl textual values to numerical ones used in Api\Mail\Account
 		foreach(array('acc_imap_ssl', 'acc_sieve_ssl', 'acc_smtp_ssl') as $name)
 		{
-			switch(strtolower($data[$name]))
+			switch(strtolower($data[$name] ?? ''))
 			{
 				case 'no':       $data[$name] = Api\Mail\Account::SSL_NONE; break;
 				case 'starttls': $data[$name] = Api\Mail\Account::SSL_STARTTLS; break;
@@ -562,4 +562,4 @@ class setup_cmd_config extends setup_cmd
 			}
 		}
 	}
-}
+}
\ No newline at end of file
diff --git a/setup/inc/class.setup_cmd_header.inc.php b/setup/inc/class.setup_cmd_header.inc.php
index 67e430004d..92129ee5ed 100644
--- a/setup/inc/class.setup_cmd_header.inc.php
+++ b/setup/inc/class.setup_cmd_header.inc.php
@@ -341,7 +341,7 @@ class setup_cmd_header extends setup_cmd
 		}
 		elseif ($value !== '')
 		{
-			self::_set_global(str_replace('@', $domain, $type), $name, $value);
+			self::_set_global(str_replace('@', $domain ?? '', $type), $name, $value);
 			if ($type == 'egw_info/server/server_root')
 			{
 				self::_set_global('egw_info/server/include_root', $name, $value);
diff --git a/setup/inc/class.setup_detection.inc.php b/setup/inc/class.setup_detection.inc.php
index 0744ba36c3..f57a3c7fbc 100755
--- a/setup/inc/class.setup_detection.inc.php
+++ b/setup/inc/class.setup_detection.inc.php
@@ -199,7 +199,7 @@ class setup_detection
 						{
 							$major_depsvalue = $GLOBALS['egw_setup']->get_major($depsvalue);
 							$tmp = explode('.',$depsvalue); $minor_depsvalue = array_pop($tmp);
-							$tmp2 = explode('.',$currentver); $minor = array_pop($tmp2);
+							$tmp2 = explode('.', $currentver ?? ''); $minor = array_pop($tmp2);
 							if ($major == $major_depsvalue && $minor <= $minor_depsvalue)
 							{
 								$setup_info['depends'][$depkey]['status'] = True;