Creating an XMLHttpRequest wrapper function

I have been trying to use AJAX calls to php to do various tasks, and reading the material on it at w3schools.com, where it mentions that if you are using more than one function with an XMLHttpRequest object, that you should create a single function which creates the Object, and pass it parameters including the onr开发者_Go百科eadystatechange function. The example they then offer doesn't seem to actually do this, and I seem to be getting garbage when I have 2 request objects functioning at the same time. Can somebody either point me to a better instructional website or give me the short and dirty on how to create the wrapper function?