一种向javascript调用的webservice添加访问控制的方法？

I have a PHP webservice that I need to add exclusive access to. This I could do with md5sums or similar, but my problem is that I also have a website that calls the webservice from javascript. So any keys, md5sums, and what I can think of can be read in the javascript and does not provide much security. For example an url like I have a PHP webservice that I need to add excl