PHPBuilder.com: Four Sane Solutions for PHP Debugging
Few tasks are more tedious and frustrating than debugging a Web application. [...] Fortunately, PHP developers have a number of powerful debugging solutions at their disposal. Whether you're merely inspecting array contents or attempting to determine the status of an Ajax-driven POST response, these four solutions are guaranteed to have an immediate impact on your productivity.
His four solutions involve changing the error reporting level on your development environment higher than production to catch issues that might slip through unnoticed, using XDebug, integrating FirePHP and using test-driven development to be sure things work from the outset.