If you write web applications in PHP for a living or or you have tackled a big project then you most likely would be interested PHP namespaces in version 5.3. This article does a great job at explaining what a namespace is and how to utilize them in PHP.
PHP’s historical lack of namespace support has long been a sore spot for developers residing both within and without the PHP community. Lacking the ability to efficiently organize project libraries, PHP developers have long made do with various contrived hacks, including notably using an autoloader in conjunction with code organized within a nested folder structure. Such an approach is used by the PHP Extension and Application Repository (PEAR), as well as by popular frameworks such as the Zend Framework.