While preparing my "PHP Under The Hood" talk for the Dutch PHP Conference there was a question on IRC about extension_loaded() being faster than function_exists(), which might be strange as both of them are simple hash lookups and a hash lookup is said to be O(1). I started to write some slides for it but figured out that I won't have the time to go through it during that presentation, so I'm doing this now.

He talks about array storage (a "real" array), numeric and string-based keys, the internals of how each is stored and how the differences make the one function faster than the other (hint: it's all about collisions).

Unless you're creating very simple applications, you will soon require one or more PHP extensions. Extensions are code libraries which extend the core functionality of the language. [...] What happens when you want to move your web application to another host or platform where a different set of extensions are configured?

Using the extension_loaded function built into PHP, you can create intelligent code that can fall back on a different technology if needed. In his example its trying to check for the GD graphics extension and echoing and error message if it's not found. The function_exists function can be used similarly.

As a GHOP Task , Cornil did a performance analysis of Drupal and found its two largest performance drains were the bootstrap process and the theming layer. Quite simply, Drupal spends too much time including code. [...] Fortunately, Drupal 7's self-learning code registry system has just landed, which should obliterate most of the wasted bootstrap cost.

Larry describes the "heart of it all", the token_get_all call, that parses through an entire PHP file, splitting out things like classes included and functions called. This is passed through a function_exists call to the current script and, if it's already there, the file isn't included repetitively.

is_callable() will return TRUE even if a function has been disabled in php.ini with disabled_functions (which, in my mind, is contrary to what '�is_callable'� implies). function_exists() will return FALSE if the function is disabled in php.ini, but will return TRUE if the function is being blocked by safe_mode.

So, to help counter these issues, he checks first to see if the exec('id') option works - checking it work function_exists and the setting for safe_mode (disabled). If it doesn't pass, he moves on to the posix_* function and tests then with a function_exists.