The xml.it blog passes on a quick tip for something that was exactly what they were looking for:

We were looking for a simple open-source PHP profiler, without any fancy IDEs like Zend bundled together. After a little search, we found one: Xdebug, debugger and profiler tool for PHP.

XDebug is "extension helps you debugging your script by providing a lot of valuable debug information. The debug information that Xdebug can provide includes the following: stack traces, memory allocation and protection for infinite recursions".