PHP.net has excellent basic resources. The manual is handy.
As for the other: https://www.google.com/search?aq=f&sourceid=chrome&ie=UTF-8&q=multi+tier+architecture+in+php has some good links to read.
I'm not sure we're large enough to be classed as "enterprise". Our RDBMS is MySQL, accessed via John Lim's ADODB. Our business logic and interface are, most likely, considered "classified" (and that's ironic considering what we do) but are based on a commercial PHP product which I'm not really that impressed with, but, then, we're using a older version and have too many "hacks" installed to do any updating with the resources currently available.
I would suggest that many PHP enterprises have a lot of homegrown stuff; of course, there's Drupal, PHP-nuke, etc., as well.
Good luck, and welcome to PHPBuilder!