For the FileMaker API for PHP, it's generally preferred that you learn some PHP first. If you don't know any, I would suggest codecademy's course to get started:
If you already know PHP, then the following were articles designed for PHP developers to pick up on the FileMaker API classes:
The above PDFs are a little dated but contain the basics.
There are plenty of other online resources (IE Lynda.com) and books (IE http://www.amazon.com/FileMaker-API-PHP-Todd-Duell/dp/0615718078) to help you out as well once you pick up some PHP.
I think Jonathan Stark's book "is a MUCH better resource for learning the API. (I've read them all.)
Web Publishing with PHP and FileMaker 9
Don't let the "FileMaker 9" in the title fool. All content still applies, and this is the fastest and easiet way to come up to speed that I've found.
I assume the API has not changed much since FM 9, right?
Soimewhere between not much, and not at all... somewhere along the line they added a couple of minor things (I forget exactly when) but since 11 there have been no changes other than the correction of deprecation errors (and 12 and 13 API code is identical)
I'd agree - this is a great resource...