Is PHP Thread-Safe, and Does It Matter for Web Server Performance?

What Defines Thread Safety and Non-Thread Safety in PHP?

Background on Concurrency Approaches:

Different web servers utilize various methods for parallel handling of HTTP requests. Popular approaches include threads and processes. Apache HTTP server supports both the worker MPM (threads) and prefork MPM (processes) models.

PHP's Integration with Web Servers:

PHP integrates with web servers through methods like mod_php, where PHP is loaded within the web server. When Apache uses threads for concurrency, it necessitates PHP's thread-safety.

Thread Safety in PHP:

PHP's thread-safety has been a subject of debate. It's generally recommended to use thread-safe PHP when embedding it in a multi-threaded server like Apache's worker MPM. However, PHP's thread-safety remains disputed.

Recommendation:

If possible, it's advisable to avoid using PHP in multi-threaded environments due to its questionable thread-safety.

Additional Notes:

• Most Linux distributions use the prefork MPM for Apache, eliminating the thread-safety concern.
• PHP thread-safety is irrelevant when using other web servers like nginx or lighttpd.
• The PHP version used for command-line operations is also unaffected by thread safety.
• The optimal choice between thread-safe and non-thread-safe PHP remains uncertain, but the non-thread-safe version is likely more efficient or has fewer bugs.

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