One issue you will be faced with is 'Cross site Scripting' with Javascript / AJAX.
You can read up on it a bit here:
http://snook.ca/archives/javascript/cross_domain_aj
Also, thinking your process through, you will need sufficient javascript code to create a 'widget' on any place, and have a way to communicate BACK to your server (keep in mind PHP only runs local on your machine, so it cannot be used remotely in your javascript).
You will probably need to build a JSON API (google / stack search this if needed).
And enable communication from your JAVASCRIPT to the API (don't think of it as to PHP, even tho php will be your API server side language).
Here is an example of a PHP JSON API (on youtube too):
http://www.youtube.com/watch?v=F5pXxS0y4bg