foreach ($elements[0] as $current) {
    $file_handler = fopen($my_folder . "/" . $current . ".txt", "a");
    $locked = flock($file_handler, "LOCK_EX");
    while (!$locked) {
        usleep(500000);
        $locked = flock($file_handler, "LOCK_EX");
    }
    //got lock
    fwrite($file_handler, $mystring . "n");
    //release lock
    flock($file_handler, LOCK_UN);
    fclose($file_handler);
}
return;

$locked = flock($file_handler, LOCK_EX);