PHP 3 includes support for a network-based debugger.
PHP 4 does not have an internal debugging facility.
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://dd.cron.ru/dbg/, the
Advanced PHP Debugger (APD) or
Xdebug which even has a compatible
debugger interface as PHP 3's debugging functionality as is described
in this section.