XHProf is a PHP profiler originally written by Facebook, to provide a more lightweight alternative to XDebug.
After installing the
xhprof PHP module, profiling can be enabled / disabled from PHP code:
xhprof_enable(); doSlowOperation(); $profile_data = xhprof_disable();
The returned array will contain data about the number of calls, CPU time and memory usage of each function that has been accessed inside
xhprof_sample_disable() can be used as a more lightweight option that will only log profiling information for a fraction of requests (and in a different format).
XHProf has some (mostly undocumented) helper functions to display the data (see example), or you can use other tools to visualize it (the platform.sh blog has an example).