Xdebug

 ( Official Website ) 

Xdebug is a PHP extension which provides debugging and profiling capabilities.

The debug information that Xdebug can provide includes the following:

• Stack trace and function traces in error messages
  (2009). 9780596101015, O'Reilly Media. ISBN 9780596101015
  with:

*full parameter display for user defined functions

*function name, file name and line indications

*support for member functions

• memory allocation
• protection for Infinite loop

Xdebug also provides:

• profiling information for PHP scripts
  Cal Henderson (2006). 9780596102357, O'Reilly Media. ISBN 9780596102357
• code coverage analysis
• capabilities to debug your scripts interactively with a debugger front-end.
  (2007). 9780471776130, John Wiley & Sons. ISBN 9780471776130

Xdebug is also available via the PECL.

• Debugger
• Dynamic program analysis
• Software performance analysis
• Optimization
• DBG (another open source PHP debugger)
• Zend Studio (the Zend Debugger is an alternative to Xdebug)

• Xdebug in PECL
• Xdebug trace log visualization tool