Your belief that RETURNING doesn't work in Oracle is untrue. It does (though you have to RETURNING a .nextval, indeed), and is the recommended approach rather than using .currval, because it saves a roundtrip to the database.
Writing an abstraction layer which abstracts over all SQL differences is of course over the top... but abstracting over trivial, always-needed stuff like 'what is the sequence number of the row I just inserted' is, well, trivial. I've written such an abstraction layer myself more than once, and that was a little one-man thing, and it was not very much effort. That PHP couldn't do it says volumes.