New on PHPBuilder.com today there's a tutorial on profiling your applications with the help of XDebug and the KCacheGrind tools.

[Eventually you'll want to seek out and resolve any bottlenecks.] Which brings us to the question of how to even go about determining which parts of an application could conceivably be optimized. One common approach involves using a profiler such as Xdebug, which can analyze your code and produce performance reports. These reports can then be reviewed within a profiling visualization tool such as KCacheGrind. In this article I'll show you how to use Xdebug and KCacheGrind to begin profiling and analyzing your PHP-driven Web applications.

He starts you from the beginning - installing both XDebug and KCacheGrind (unix-based, if you're on Windows try WinCacheGrind). He gives an example of a profile report based on a Zend Framework application and an example call graph output (as well as how some caching can help things quite a bit).