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.