XHProf is a function-level hierarchical profiler for PHP and has a simple HTML based user interface. The raw data collection component is implemented in C (as a PHP extension). The reporting/UI layer is all in PHP. It is capable of reporting function-level call counts and inclusive and exclusive wall time, CPU time and memory usage. Additionally, it supports ability to compare two runs (hierarchical DIFF reports), or aggregate results from multiple runs.  Originally developed at Facebook, XHProf was open sourced in Mar, 2009.