On the Experimentalworks blog a recent post looks at using DTrace, the powerful tool to debug and trace problems in an application, with your PHP applications.
Dtrace is really powerful and trying to do an introduction to all it's features is just not possible. Therefore I will just focus on the basics, that are needed to get our stuff working. The basic idea behind Dtrace is that the kernel and userland programs fire probes on a specific location in the kernel or the userland program.
He looks at the structure of a DTrace program as well as a list of probes that you can use to help track down the problem in your application. His "first example" traces the compile time of the app and, when run on the command line, returns the time in seconds.