Sameer Borate has posted some of his thoughts about logging in PHP applications and how it can be one of the most important things you do.

Software development is an exercise in managing complexity, and the more tools and methods we have at our disposal the easier it gets. One of my favorites is logging - recording important code execution points that I think are important, especially during debugging. [...] But what information does one log and what does one keep out. This usually depends on what kind of application you are developing.

He gives an example of logging on database intensive applications - storing the query execution state - complete with a simple example of an "addUser" method that logs any issues caused by the query execution. If you're not using a framework that has it's own logger, he recommends a few: KLogger, Log4PHP and the Zend_Log component of the Zend Framework (can be used separately)