I’ve used a lot of the PHP API libraries, and they all allow you to enter the API version as part of the URL. For example, change:
1/users/show
to:
1.1/users/show
That’s all it takes to make it work with the 1.1 API. You’ll also have to verify that the data returned is the same structure/member names as for 1.0, but that is independent of the library.