public static function check($filename)
    {
        if (!$filename = static::path($filename)) {
            return null;
        }

        if (!file_exists($filename)) {
            return null;
        }

        return filemtime($filename);
    }
public static function check($filename)
    {
        if (!$filename = static::path($filename)) {
            return null;
        }

        if (!file_exists($filename)) {
            return null;
        }

        return filemtime($filename);
    }
public static function check ( $ filename ) { if ( ! $ filename = static :: path ( $ filename ) ) { return null ; } if ( ! file_exists ( $ filename ) ) { return null ; } return filemtime ( $ filename ) ; }
Check if the specified key has already been cached.
If it has been cached then the time the cached data was last modified is returned.

@param string $filename The key of the cached data

@return int|null
Check if the specified key has already been cached . If it has been cached then the time the cached data was last modified is returned .