Larry Garfield talks about a new feature of Drupal 7 in a new post to his blog - the new introspective code registry that's been introduced in this latest version.

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.

In this new post to his blog, Ed Finkler talks about some tests he worked up to discover if a function is available in PHP or not while working on the latest version of PHPSecInfo).

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.