PHP 5.6.0 alpha finally ships with an interactive debugger called phpdbg.
phpdbg project page: http://phpdbg.com/
PHP 5.6 is shipped with the phpdbg interactive debugger. PHP versions below 5.6 do not have an internal debugging facility though. You can use one of the external debuggers though. The » Zend IDE includes a debugger, and there are also some free debugger extensions like DBG at » http://www.php-debugger.com/dbg/, the » Advanced PHP Debugger (APD) or » Xdebug.