whilst not being a big fan of PEAR (in general) ... or any abstraction layer for DB work I suspect that PEAR may win in the long run as `THE` DB layer though not because it is any better ... possibly the opposite...
I would suggest you think about writing seperate Database classes for each database you think you may ever require, some DB abstractions can work, but to get an abstration layer that works from the bottom to the top of the scale often gives very poor, innefficient code , want proof ... look at PEAR
In general, if you think you will be able to port a MySQL app to Oracle then you probably have problems ahead

aside from SQL syntax which is hard to write for both, Oracle requires a totally different methodology to get the best out of compared to MySQL inless you are only using Oracle at the same level as MySQL in which case stick to MySQL cos its faster and cheaper