Big airports are using Oracle, it's the fastest databases (I think)... Oracle products are not freeware ;-) or even cheep..... They are damn expensiv!
Manny danish banks are using some kind of IBM SRS32 systems (manny million software) and i know that Lotus have made some quite fast database software....
If you was you I would stick to Oracle's software, I just don't know how you sould get et legally... illegal software on servers are crap!
(don't ask about how to integrade php with oracle, I don't know any thing about it.... Just that Oracle are using SQL too).