PHP mutual exclusion (mutex)

I recently created my own simple implementation of a mutex-like mechanism using the flock function of PHP. Of course the code below can be improved, but it is working for most use cases.

function mutex_lock($id, $wait=10)
{
  $resource = fopen(storage_path("app/".$id.".lck"),"w");

  $lock = false;
  for($i = 0; $i < $wait && !($lock = flock($resource,LOCK_EX|LOCK_NB)); $i++)
  {
    sleep(1);
  }

  if(!$lock)
  {
    trigger_error("Not able to create a lock in $wait seconds");
  }

  return $resource;
}

function mutex_unlock($id, $resource)
{
  $result = flock($resource,LOCK_UN);
  fclose($resource);

  @unlink(storage_path("app/".$id.".lck"));

  return $result;
}

The question is: Where will you store the stats that the FTP servers are pushing with POST to your update.php file? If it's a local file, than ircmaxell in the second post has answered you. You can do this with a mutex as well - the semaphore functions. Another solution is to use MySQL MyISAM table to store the stats and use something like update info_table set value = value + 1. It should lock the table, and serialize your requests, and you will have no problems.

Well, most of PHP runs in a different process space (there are few threading implementations). The easy one is flock. It's guaranteed to work on all platforms.

However, if you compile in support, you can use a few other things such as the Semaphore extension. (Compile PHP with --enable-sysvsem). Then, you can do something like (note, sem_acquire() should block. But if it can't for some reason, it will return false):

$sem = sem_get(1234, 1);
if (sem_acquire($sem)) {
    //successful lock, go ahead
    sem_release($sem);
} else {
    //Something went wrong...
}

The other options that you have, are MySQL user level locks GET_LOCK('name', 'timeout'), or creating your own using something like APC or XCache (Note, this wouldn't be a true lock, since race conditions could be created where someone else gets a lock between your check and acceptance of the lock).

Edit: To answer your edited question:

It all depends on your server configuration. PHP May be run multi-threaded (where each request is served by a different thread), or it may be run multi-process (where each request is served by a different process). It all depends on your server configuration...

It's VERY rare that PHP will serve all requests serially, with only one process (and one thread) serving all requests. If you're using CGI, then it's multi-process by default. If you're using FastCGI, it's likely multi-process and multi-thread. If you're using mod_php with Apache, then it depends on the worker type:

1. mpm_worker will be both multi-process and multi-thread, with the number of processes dictated by the ServerLimit variable.
2. prefork will be multi-process
3. perchild will be multi-process as well

Edit: To answer your second edited question:

It's quite easy. Store it in a file:

function readStatus() {
    $f = fopen('/path/to/myfile', 'r');
    if (!$f) return false;
    if (flock($f, LOCK_SH)) {
        $ret = fread($f, 8192);
        flock($f, LOCK_UN);
        fclose($f);
        return $ret;
    }
    fclose($f);
    return false;
}

function updateStatus($new) {
    $f = fopen('/path/to/myfile', 'w');
    if (!$f) return false;
    if (flock($f, LOCK_EX)) {
        ftruncate($f, 0);
        fwrite($f, $new);
        flock($f, LOCK_UN);
        fclose($f);
        return true;
    }
    fclose($f);
    return false;
}

function incrementStatus() {
    $f = fopen('/path/to/myfile', 'rw');
    if (!$f) return false;
    if (flock($f, LOCK_EX)) {
        $current = fread($f, 8192);
        $current++;
        ftruncate($f, 0);
        fwrite($f, $current);
        flock($f, LOCK_UN);
        fclose($f);
        return true;
    }
    fclose($f);
    return false;
}