David Parra has a new post to his site today about a method of using Systemtap to profile PHP as the code executes (as an alternative when DTrace may not be available.

PHP introduced DTrace support with PHP 5.3, enabling probing points in the PHP executable that can be used to simplify probing of PHP applications without having to the PHP implementation details. We enabled probes on function calls, file compilation, exceptions and errors. But this has always been limited to the operating systems that support DTrace. With the popularity of DTrace, Systemap programmers decided to add a DTrace compatibility layer that allows to use DTrace probes as Systemtap probing points as well.

Thanks to a recent commit to the PHP 5.5 branch, your PHP installation (compiled with DTrace support) can be executed with the "stap" command and searched for probe points. He includes a simple Systemtap script that counts the calls of a specific function to get you started.