System-wide waiting queue in PHP

I have put together a few functions in PHP that allow processes to enter into a queue and wait their turn. This queue is file based and thus accessible to all processes on the system. The use case is e.g. a function call or processing of a file that should not have more than one instance on the whole system.

There are two files, one that contains the actual queue (queue.php) and a lock for accessing the queue (queue.lock). All the functions are part of a bigger PHP object, hence the private modifiers and occasional $this references.

As of now I can't get any kind of indention to work here, sorry for that. Now for the function that lets a process put his ID into the queue:


private function push_to_queue()
{
$queue = Array();
$fh = fopen($this->folder."/queue.lock","w");
$start_time = microtime(true);

do
{
$haveLock = flock($fh, LOCK_EX);
if(!$haveLock) usleep(rand(0,100)*10000);
}
while(!$haveLock && (microtime(true)-$start_time) < 10000);

if($haveLock)
{
include($this->folder."/queue.php");

if($queue == null) $queue = Array($this->id);
else array_push($queue, $this->id);

$save = '$queue = unserialize(\''.serialize($queue).'\') ?>';
file_put_contents($this->folder."/queue.php", $save);
}

fclose($fh);
}

Next the function which lets a process remove his id from the queue:

private function remove_from_queue()
{
$fh = fopen($this->folder."/queue.lock","w");

$start_time = microtime();

do
{
$haveLock = flock($fh, LOCK_EX);
if(!$haveLock) usleep(rand(0,100)*1000);
}
while(!$haveLock && (microtime(true)-$start_time) < 10000);

if($haveLock)
{
include($this->folder."/queue.php");

$key = array_search($this->id,$queue);

if($key !== FALSE)
{
unset($queue[$key]);
$queue = array_values($queue);
}

$save = '$queue = unserialize(\''.serialize($queue).'\') ?>';
file_put_contents($this->folder."/queue.php", $save);
}

fclose($fh);
}

And last a function for reading the queue:

private function load_queue()
{
$fh = fopen($this->folder."/queue.lock","w");

$start_time = microtime(true);

do
{
$haveLock = flock($fh, LOCK_SH);
if(!$haveLock) usleep(rand(0,100)*1000);
}
while(!$haveLock && (microtime(true)-$start_time) < 10000);

if($haveLock)
{
include($this->folder."/queue.php");
return $queue;
}

fclose($fh);
}

Usage for the functions currently looks like this:

$this->push_to_queue();

$sleep_time = 0;

$queue = $this->load_queue();

while($queue[0] != $this->id)
{
usleep(500000);
$sleep_time++;

if($sleep_time >= 120)
{
$return = Array("error" => "queue_timeout");
$this->make_log("[WARNING] Request timed out in queue.");
return $return;
}

$queue = $this->load_queue();
}

// DO STUFF

$this->remove_from_queue();

Caveats: Obviously there could be a lot of disk access with the file based queue.