In a recent post from Learncomputer.com, there's a comparison of two PHP profilers - the PHP Quick Profiler and the one included in Xdebug.

Whether you are an experienced developer or just getting started it is important to know how to measure the performance of your scripts and applications so that you can learn to make improvements and optimizations to your code. [...] This article compares two of the most popular [profiling] solutions under free license that you can begin using today to profile your PHP applications.

They describe each of the tools - the Quick PHP Profiler acting more like a plugin (running on each page load) and Xdebug working more behind the scenes and providing cachegrind files. These files can be viewed in cachegrind tools to drill in to the badly performing aspects of your applications and find the issues.

If you need a free tool it can be difficult to find a PHP profiling tool that has all of the features you may want and the interface that you like all rolled into one. Identifying what kind of data you are looking for and what information you need from a profiling tool will allow you to choose the best solution for your needs.