Mike Naberezny has a new post today about one of the easiest ways he's found to "wrap up" those functions in your code that make the unit testing for your application break when you run it:

One of the problems that hampers the testability of PHP code is the coupling created by accessing all of the PHP global functions. This happens often because a large number of useful extensions are accessed only through global functions.

He gives the example of an LDAP connect script that, because it requires an external LDAP server to connect to and return a response from, would break the tests. His way around the problem is to use an extension versus just calling the object itself (code included) through a ExtensionProxy. Then, it's a simple matter of calling the proxy object with the type parameter for the function to be called and evaluating the response of the proxy instead of the actual call.