Xdebug is a PHP extension which provides debugging and profiling capabilities. It uses the DBGp debugging protocol.
The debug information that Xdebug can provide includes the following: stack and function traces in error messages with: full parameter display for user defined functions function name, file name and line indications support for member functions memory allocation protection for infinite recursions
Xdebug also provides: profiling information for PHP scripts code coverage analysis capabilities to debug your scripts interactively with a debugger front-end.
Xdebug is also available via the PECL.
See also
Debugger
Dynamic program analysis
Software performance analysis
Optimization
DBG (another open source PHP debugger) Zend Studio (the Zend Debugger is an alternative to Xdebug)