isValidKey() fix for PHP < 5.4

PHP < 5.4:
"Fatal error: Can't use function return value in write context in (...)"

Author: KKKas

src/Enum.php

@@ -119,7 +119,8 @@ public static function isValid($value)
      */
     public static function isValidKey($key)
     {
-        return isset(self::toArray()[$key]);
+        $array = self::toArray();
+        return isset($array[$key]);
     }
 
     /**