On PHPBuilder.com today there's a new article from Jason Gilmore introducing you to namespaces in PHP 5.3+ development. Namespaces make it simpler to separate out your code into functional pieces and help keep it organized.
The inclusion of namespace support within PHP 5.3 effectively brought the need for gripes and workarounds to a halt, however adoption of this exciting new feature has seemed surprisingly slow in the more than two years since its release. [...] The utility of this new feature is simply undeniable. Therefore I thought it would be worthwhile to offer a formal introduction to namespaces for the benefit of those PHP developers who haven't yet had the opportunity to investigate the topic.
He starts by introducing the concept of a "namespace" as a sort of container for your code, providing separation that prevents errors like the infamous "cannot redeclare class" issue. He includes examples of PHP's namespace syntax to split out two "Account" classes into two different sections. Using them is as easy as referring to them by their namespaced "path" or using something like the "use" keyword to reassign it to another name.