DB portability

I am looking at undertaking a very large project that will be handled initially in MySQL. Down the road we may choose a corporate db such as Oracle. My thought was since we will have thousands of lines of code to migrate to the new db, I could create a global script that is required by each individual script. This global script would alias typical database functions (connects, queries, etc...) to PHP's MySQL functions for the time being and I could adjust the alias in one central location if we switch from MySQL (I never foresee this happening but you never know). Can any of you see any repercussions of this central database alias script? -NC