Database Code Portability

I haven't seen this issue discussed, sorry if I missed it: My question is why are the PHP database functions so unportable? Is there a reason why they have to be specific to, say MySQL. It seems to me (even though I am an amatuer to this stuff) that the code should be much more easily portable from one db to another? Do most coders add an abstraction layer themselves? Or maybe people just don't port PHP code much and so don't worry about it? Mainly curious.... Dave