When it comes to multi-tasking and interacting with the operating system, PHP is no lightweight. In this December 6, 2012 presentation Jonathon Hill (@compwright) explores PHP's little-known POSIX, Process Control, and Semaphore extensions and shows you how to introduce a parallel dimension to your applications.

His presentation covers the use of multi-tasking to accomplish some common tasks in PHP (like batch processing or creating daemons). He talks about multi-processing vs multi-threading, forking, race conditions and semaphores.

You can watch the video of the presentation over on Vimeo and check out the software and links to other resources mentioned in the talk in this link bundle.

He lists five different pain points he discovered when trying to use the extension:

• Lack of a true Semaphore
• Undefined error handling
• Undefined behavior of sem_get()
• Cannot disable semaphore auto-releasing
• A semaphore may be deleted when other processes are waiting to acquire it

The semaphore extension provides a PHP-based wrapper for the System V IPC family of functions (including semaphores, shared memory and inter-process messaging).

I've recently had a problem with a rather large operation (that could probably be optimised considerably, but nevermind), where by if the cached result of the operation expired, several web server threads would attempt the operation, causing some major headaches for our database and web servers. This is something I've come across before, and is commonly(?) known as a Cache Stampede. This is bad, this post describes the basics of what I've done to deal with it.

His example uses a Zend_Cache interface to Memecache, but it could be applied in other places too. His method uses a binary semaphore to check and see if there's a "lock" and sleep for a bit before checking again. Sample code is included showing how to create this system and how to refresh the data with a new expiration on a cache miss.

If you get an error when PHP launches along the lines of: "PHP Warning: PHP Startup: mm_create(0, /var/www/phpSessionStorage/session_mm_cli1000) failed, err mm:core: failed to open semaphore file (Permission denied) in Unknown on line 0" you're going to either need to upgrade to PHP 5.3.0 or wait for PHP 5.2.12.

It seems that some debugging code made it into the core code (but it's been corrected in the branches).

• Sandboxed semaphores
• Focus on *printf [continued]
• PHP 5.2.1 RC2 and PHP 4.4.5 RC1
• It's all in the HEAD

Be sure to check back each week for a new summary.