PHPBuilder.com has posted a list of resources that they offer to help both beginning and experienced PHP developers to further their knowledge:
PHP is one of the most popular scripting languages used to develop applications on the web today. As a result, internet.com has a multitude of PHP resources throughout our network of websites. Here are some of our best PHP resources, along with some featured tutorials and out-of-network resources that you may not know about.
The grouping of links also include external resources like the main PHP site and Zend's website.