Xcode debugger

For advanced development a debugger may be very useful. Xdebug is the standard tool.

The main site:

http://xdebug.org/

Some articles on setting up and using it:

http://devzone.zend.com/article/2803-Introducing-xdebug

http://community.activestate.com/faq/how-do-i-get-php-debuggin

This Drupal module has additional tools for visualizing the Xdebug call traces:

http://drupal.org/project/visualize_backtrace