Subquery in MySQL using php logic

[admin]

I've looked around online for a while for a some kind of API that will allow me to use PHP logic to perform subqueries in MySQL. I want the syntax to be identical to that of other sql servers, like Postrgres, so that if I want to change backends, I can just pull the special PHP logic, and continue on my merry way. I haven't found any logic out there like this. I was about to sit down and start working on that, but being such a desirable tool, I figure if it can be done, it'd be out there.
Does anyone have any comments, sudgestions, or code?

Thanks,
Matt