DTrace and PHP:

"PHP core developer Wez Furlong was busy adding a DTrace provider to PHP. After a little bit of work (and a little bit of debugging), we got it working -- and damn is it cool. Wez implemented it as a shared object, which may then be loaded via an explicit extension directive in php.ini. Once loaded, two probes show up: function-entry and function-return. These probes have as their arguments a pointer to the function name, a pointer to the file name, and a line number."

(Via The Observation Deck.)