On the SitePoint PHP blog today Craig Buckler has a suggestion on how to handle unloaded extensions in your application in case you need to define a failover.

Unless you're creating very simple applications, you will soon require one or more PHP extensions. Extensions are code libraries which extend the core functionality of the language. [...] What happens when you want to move your web application to another host or platform where a different set of extensions are configured?

Using the extension_loaded function built into PHP, you can create intelligent code that can fall back on a different technology if needed. In his example its trying to check for the GD graphics extension and echoing and error message if it's not found. The function_exists function can be used similarly.