#php: write log to file txt

private function wlog($str, $file_name){
        $file = isset($file_name) ? $file_name : 'default_log.txt';
        $current    = $str . PHP_EOL;
        // Write the contents to the file
        $f = fopen($file, 'a');
        fwrite($f, $current);
        fclose($f);
    }