I am looking for an alternative to the Pear DB to allow my PHP application to use different SQL servers by simply specifying a "driver" file, or something similar. The key is that I do not want to have to install Pear on the server(s) the application will be on. Any advice is greatly appreciated.

I prefer ADODb. You can install either in your application's directory if you want to, it's not difficult with PEAR either, but ADODb is designed to work this way, and I prefer it anyway.

Check out adodb-lite. I use it in all of my projects and it's been a great experience. You can load up some features you might need and leave out others.

http://adodblite.sourceforge.net/index.php