Zend, the company that makes the PHP engine, offers certification for users who are interested in measuring their PHP expertise and qualifying their PHP skill level. From Zend's FAQ:
Becoming PHP certified is a measure of your experience in the world of PHP. The goal of the Zend PHP Certification program is to allow PHP professionals to attain the "Zend Certified Engineer" designation. This is identical in concept to other well know certification programs such as "Microsoft's Certified Professionals (MCP)" and the "Sun Certified Java Programmer (CSJP)" programs.
The Zend PHP Certification program aims at creating a measure of distinction that employers can use to evaluate prospective employees. Becoming a Zend Certified Engineer will not make you a better programmer, however, it will most likely improve the chances of your resume standing out from among other applicants vying for the same position.
You are tested on your fundamental knowledge of the PHP language, as well as various classes used in PHP and design patterns PHP developers should be aware of.
I took the certification test this past Monday and passed the examination. I am now officially a "Zend Certified Engineer".