On the Think Vitamin blog Kieran Masterson has put together an article about debugging PHP applications - everything from error levels out to a few useful tools that can help with some of the more complex issues.

Nobody enjoys the process of debugging their code. If you want to build killer web apps though, it’s vital that you understand the process thoroughly. This article breaks down the fundamentals of debugging in PHP, helps you understand PHP's error messages and introduces you to some useful tools to help make the process a little less painful.

He looks at changing the display_errors setting, updating the error_reporting level, the types of errors (notice, warning, etc) and the Xdebug and FirePHP tools.