AFAIK = As far as I know
I'm with Tango here. Expertise or proficiency would be how I may classify it. You are a PHP developer with expertise or proficiency with Joomla CMS for example. That would tell me you know and have experience coding joomla extensions. I would classify myself as a PHP developer with an expertise in OO implementation. I don't develop in Joomla or any CMS, but that doesn't make me incapable of doing so.
The only real classification to a language is your experience in using it (for which I will take from the book of Skyrim
): Novice, Apprentice, Adept, Expert, Master. I would classify myself as an Expert PHP Programmer due to my understanding of the language.
. . .I'd love to see a resume that says Adept PHP Programmer. Lol.
If you are Zend Certified, you are simply a ZCE (Zend Certified Engineer). I don't have my ZCE, but from what I can tell of the exam, you can become certified with an 'Adept' skill level.