Can I have JS call my ASP/PHP/Perl function?
[server]--/ ~ /--[client]
The answer to that question lies in this story:
User Computer A asks for your web page. Computer A is in California, and makes a request over the Internet for the file from your web server, Server Z.
Server Z sits in Florida, running a web server with the server-side language of your choice (PHP, for the sake of this discussion).
Server Z gets the request, and creates the web page by combining bits of raw text with the output of your server-side commands. All your 'PHP' functions run on Z. When it's done, it has an HTML file (possibly with CSS and JS in it) ready to go.
Server Z sends this file over the Internet back to Computer A. When it's done, Server Z can *explode* and nothing about the following will change.
Computer A gets the HTML+CSS+JS file, and the web browser there renders the HTML code and runs the JS code. Computer A doesn't know anything about PHP. When, 45 seconds after receiving the page, the user changes a <select>, the *only* way to 'run a PHP function' is for the Javascript code on Computer A to make a new HTTP request to Server Z, asking for the information.
Once you understand the above, ask the channel about XMLHTTP, hidden iframes, or images/cookies for ways to communicate with the server without reloading the page.