diff --git a/api/src/Mail/hash_pbkdf2.php b/api/src/Mail/hash_pbkdf2.php
index fe3650091a..78a1d53072 100644
--- a/api/src/Mail/hash_pbkdf2.php
+++ b/api/src/Mail/hash_pbkdf2.php
@@ -5,21 +5,25 @@
  * @author Sebastiaan Stok <s.stok@rollerscapes.net>
  * @link https://github.com/symfony/polyfill-php55/blob/master/Php55.php
  */
-function hash_pbkdf2($algorithm, $password, $salt, $iterations, $length = 0, $rawOutput = false)
+
+if (!function_exists('hash_pbkdf2'))
 {
-	// Number of blocks needed to create the derived key
-	$blocks = ceil($length / strlen(hash($algorithm, null, true)));
-	$digest = '';
-	for ($i = 1; $i <= $blocks; ++$i) {
-		$ib = $block = hash_hmac($algorithm, $salt.pack('N', $i), $password, true);
-		// Iterations
-		for ($j = 1; $j < $iterations; ++$j) {
-			$ib ^= ($block = hash_hmac($algorithm, $block, $password, true));
+	function hash_pbkdf2($algorithm, $password, $salt, $iterations, $length = 0, $rawOutput = false)
+	{
+		// Number of blocks needed to create the derived key
+		$blocks = ceil($length / strlen(hash($algorithm, null, true)));
+		$digest = '';
+		for ($i = 1; $i <= $blocks; ++$i) {
+			$ib = $block = hash_hmac($algorithm, $salt.pack('N', $i), $password, true);
+			// Iterations
+			for ($j = 1; $j < $iterations; ++$j) {
+				$ib ^= ($block = hash_hmac($algorithm, $block, $password, true));
+			}
+			$digest .= $ib;
 		}
-		$digest .= $ib;
+		if (!$rawOutput) {
+			$digest = bin2hex($digest);
+		}
+		return substr($digest, 0, $length);
 	}
-	if (!$rawOutput) {
-		$digest = bin2hex($digest);
-	}
-	return substr($digest, 0, $length);
-}
+}
\ No newline at end of file