Facebook developed a function-level hierarchical profiler named XHProf. In 2009 they made it open-source. However, the original PHP code is bulky, GUI has limited analytics functionality and it lacked database support. There have been a number of attempts to give this tool a new look. Sadly, those worth mentioning are limited to preinheimer-xhprof.
XHProf.io disregards any previous GUI implementations. It is a new tool to visualise data collected with the XHProf C extension.