Developed at Facebook, XHProf is a function-level hierarchical profiler for PHP and has a simple HTML based navigational interface.It is capable of reporting function-level inclusive and exclusive wall times, memory usage, CPU times and number of calls for each function. Additionally, it supports the ability to compare two runs (hierarchical DIFF reports), or aggregate results from multiple runs.
XHProf is a project hosted at PECL.
http://pecl.php.net/package/xhprof
The raw data collection component is implemented in C (as a PHP extension). The reporting/UI layer is all in PHP.