PHP mutual exclusion (mutex)

Read some texts about locking in PHP.
They all, mainly, direct to http://php.net/manual/en/function.flock.php .

This page talks about opening a file on the hard-disk!!

Is it really so? I mean, this makes locking really expensive - it means each time I want to lock I'll have to access the hard-disk )=

Can anymore comfort me with a delightful news?

Edit:

Due to some replies I've got here, I want to ask this;
My script will run only by one thread, or several? Because if it's by one then I obviously don't need a mutex. Is there a concise answer?

What exactly I'm trying to do

Asked by ircmaxell.
This is the story:

I have two ftp servers. I want to be able to show at my website how many online users are online.
So, I thought that these ftp servers will "POST" their stats to a certain PHP script page. Let's assume that the URL of this page is "http://mydomain.com/update.php".

On the website's main page ("http://mydomain.com/index.php") I will display the cumulative statistics (online users).

That's it.

My problem is that I'm not sure if, when one ftp server updates his stats while another does it too, the info will get mixed.
Like when multi-threading; Two threads increase some "int" variable at the same time. It will not happen as expected unless you sync between them.
So, will I have a problem? Yes, no, maybe?

Possible solution

Thinking hard about it all day long, I have an idea here and I want you to give your opinion.
As said these ftp servers will post their stats, once every 60sec.
I'm thinking about having this file "stats.php".
It will be included at the updating script that the ftp servers go to ("update.php") and at the "index.php" page where visitors see how many users are online.
Now, when an ftp server updates, the script at "update.php" will modify "stats.php" with the new cumulative statistics.
First it will read the stats included at "stats.php", then accumulate, and then rewrite that file.

If I'm not mistaken PHP will detect that the file ("stats.php") is changed and load the new one. Correct?

Solution

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:

mpm_worker will be both multi-process and multi-thread, with the number of processes dictated by the ServerLimit variable.
prefork will be multi-process
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;
}