Merge pull request #644 from MaSpeng/feature/624-php-8-1-compatibility

#624: Provide PHP 8.1 compatibility

Author: Sebbo94BY

src/PhpImap/Mailbox.php

@@ -486,13 +486,7 @@ public function hasImapStream(): bool
      */
     public function encodeStringToUtf7Imap(string $str): string
     {
-        $out = \mb_convert_encoding($str, 'UTF7-IMAP', \mb_detect_encoding($str, 'UTF-8, ISO-8859-1, ISO-8859-15', true));
-
-        if (!\is_string($out)) {
-            throw new UnexpectedValueException('mb_convert_encoding($str, \'UTF-8\', {detected}) could not convert $str');
-        }
-
-        return $out;
+        return imap_utf7_encode($str);
     }
 
     /**
@@ -502,7 +496,7 @@ public function encodeStringToUtf7Imap(string $str): string
      */
     public function decodeStringFromUtf7ImapToUtf8(string $str): string
     {
-        $out = \mb_convert_encoding($str, 'UTF-8', 'UTF7-IMAP');
+        $out = imap_utf7_decode($str);
 
         if (!\is_string($out)) {
             throw new UnexpectedValueException('mb_convert_encoding($str, \'UTF-8\', \'UTF7-IMAP\') could not convert $str');