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.