$fp = fopen("/tmp/lock.txt", "r+");

if (flock($fp, LOCK_EX)) { // do an exclusive lock
    ftruncate($fp, 0); // truncate file
    fwrite($fp, "Write something here\n");
    fclose($fp); /// tohel tu mema byt
    flock($fp, LOCK_UN); // release the lock
}

/// ma to by zde
//fclose($fp);