Eric Hogue has written up a new post about a few different technologies you can use to profile your PHP applications quickly and easily.
There are many available tools to profile a PHP application. Learning how to use them can help us pinpoint which parts are slow. With this information we can pick the optimizations that will give us the best results. This post describes the installation and configuration of some of them. I tested them in a Ubuntu 10.10 virtual machine. If you want to try those tools, don't forget that they can greatly impact the performance of you web server when they are active. Installing a tool like Xdebug on a production server is not recommended.
First he looks at benchmarking your application with a tool called Siege, a load testing tool that can be configured to send requests to your application in lots of different ways. He also mentions Xdebug, a handy debugger and XProf, a profiling tool to help find the bottlenecks in your code (and XHGui to view its results).