As to the first question, I see no reason why you could not use MySQL for your DBMS. For the worst case scenario that PP cannot use an ODBC connection (via a catalog) to connect to MySQL, I am sure you can schedule a bulk-export from MySQL to .txt files for PP to read.
How (or why) you would use PHP in the creation of powercubes escapes me. Are you trying to write an interface to read a powercube via an intranet? If so, I can't help you with that as I would have NO idea how to even approach that issue! ;-)
As far as I know, Cognos does only offer a "per user" type of licensing. As semby indicated, there are ways to circumvent this (multiple users sharing a single login name comes to mind) but frankly, if your company could afford the software in the first place, they should be able to afford the licences.
HTH,
John