There is a rather large list of certified PHP programmers, these are all programmers that have passed the Zend PHP certification exam.
Everyone PHP Certified in the US
you can do your own search here;
Zend PHP Yellow Pages
Hope this helps, after you've done a few gateways there pretty much all the same, (as i've done at least 9 of them now (off the top of my head)) there differences are usually laid out in a Tech Spec provided by the Gateway. Any good programmer will have no problem following these and gettting you the desired result.