PHP and javascript

How do I call PHP code that will run server side, from javascript code that is running client side? I have a lot of PHP server side code written and working within CodeIgniter. Now, my project has changed and (for reasons unimportant to this discussion) we are now NOT going to use apache and CodeIgniter, but instead, we are going to have to use an http server that does not support PHP internally. But I want to reuse my original PHP code. So I am thinking that I can execute the PHP code via a command line interface using the PHP cli interface instead of the PHP cgi interface. But, I will have to initiate this serversid call via javascript code running on the client. I know...kind of convoluted. But how would one do this if necessary? -Andres