You can take a PHP certification course through Zend. Not cheap, but well taught courses. http://shop.zend.com/en/php-training.html?hpb=mr-02-08-09-Training-Generic Most community colleges have PHP courses as well. Probably the most cost-effective way to take one, if there is a college near you.
I would recommend an online course as well. Depending on your area, it can be tough to find a course that has a good professor.
There are few college "courses" that will teach PHP and give you a certificate. Probably as it is open source and the creaters won't pay people to teach it. ASP has Microsoft paying instituations to pay for it. Are you looking for the knowledge or the certificate? For knowledge: Read PHP books For the cert: Universities teach PHP, but thats a bigger picture. Go for Zend