Profiling is a technique for observing the performance of an application, ideal for showing up bottlenecks or particularly intensive use of resources. Profiling gets inside your application and gives information about the performance of the various parts of code that are used during a request. [...] We have a choice of tools for PHP but in this article we'll focus on an excellent tool called XHGui. XHGui is a tool built upon XHProf, the profiling tool released by Facebook, but adding better storage of metrics and a much nicer interface for retrieving information from it, to the extent that it feels like a new tool in it's own right!
She walks you through the full installation, including other things the tool depends on (like MongoDB and the XHProf extension). She shows how to run the installer, configure a virtual host correctly (using mod_rewrite) and some example screenshots of what the resulting statistics report. This includes things like "wall time" and parts of the application that may be "memory hogs". There's also an example of a handy feature called the "Callgraph" that shows you a visual representation of the performance of the execution of the app.