http://nevkontakte.com/2010/Reinforced-concrete-mutexes-in-PHP.html

mutex.php

<?php
/**
 * Simple mutex implementation
 * 
 * This class implements sime kind of mutex using PHP's session file lock feature.
 * To be short, PHP forces serial access for processes trying wokr with the same session file.
 * This is the most simple mutex imitation (flock() is a strange function with unpredictable bugs),
 * but this method has some disadvantages and limitations which you should be aware of:
 * 1) Using such mutexes together with native PHP sessions can cause unexpected behaviour and possible
 *   session DATA LOSS!
 * 2) It allows not more than one locked mutex at the same time, because only one active session may exist.
 * 3) There are one known bug: when two parallel requests are handled by the same script and PHP runs as
 *   Apache's mod_php this requests will be executed in ENTIRELY SERIAL way, not only in block which are
 *   protected with mutex. I have no idea why does it happen.
 * 4) This class works only if sessions are handled by native "files" module.
 * So, this class should be used only in simple and small scripts. In large projects consider using
 * something like semaphores module (http://en.php.net/manual/en/book.sem.php)
 * 
 * @author Alek$ <aleks@aradmin.org.ru> http://nevkontakte.org.ru
 * @license New BSD License (http://www.opensource.org/licenses/bsd-license.php)
 */
class PHP_Mutex
{
	private static $mutex = null;
	
	private $locked = false;
	private $old_session = "";
	
	/**
	 * Lock mutex $id
	 * 
	 * @param string $id Mutex name.
	 */
	public static function lock($id)
	{
		self::init();
		self::$mutex->getLock($id);
	}
	
	/**
	 * Unlock mutex $id. Note that it must be the same as used in previous lock() call.
	 * 
	 * @param string $id Mutex name.
	 */
	public static function unlock($id)
	{
		self::init();
		self::$mutex->releaseLock($id);
	}
	
	private function __construct($man = false)
	{
		if(session_module_name() !== "files")
		{
			trigger_error('PHP_Mutex relies on "files" session handler module, but '.session_module_name().' is used', E_USER_WARNING);
		}
		
		if(session_id() !== "")
		{
			trigger_error('Possible session conflict with PHP_Mutex: already started session detected', E_USER_WARNING);
			$this->old_session = session_id();
		}
	}
	
	private function getLock($id)
	{
		if($this->locked === true)
		{
			trigger_error("Tried to lock mutex $id, but mutex ".session_id()." is already locked and only one locked mutex can exist at the same time", E_USER_ERROR);
		}
		
		$this->locked = true;
		session_id($id);
		session_start();
	}
	
	private function releaseLock($id)
	{
		if($this->locked == false)
		{
			trigger_error("Mutex $id isn't locked", E_USER_WARNING);
			return;
		}
		
		if($this->locked == true && $id !== session_id())
		{
			trigger_error("Tried to unlock mutex $id, but mutex ".session_id()." is currently locked", E_USER_WARNING);
			return;
		}
		
		if($this->old_session !== "")
		{
			session_id($this->old_session);
			session_start();
		}
		else
		{
			session_destroy();
			session_write_close();
			flush();
		}
	}
	
	private static function init()
	{
		if(self::$mutex == null)
		{
			self::$mutex = new PHP_Mutex();
		}
	}
}