Use Ajax to call php function with paremeters - php

Question

First off - NO JQUERY. I hate that thing with a passion and I can't possibly imagine how a native code implementation would be less efficient.
What I am after is a way to call any PHP function from Javascript and pass parameters to the function using call_user_func_array. I have written this exact code before but can't think of how I did it.
In the end, I want to be able to (in JS) be able to do something like:
var responseString = callPhpFunction(func, param1, param2, etc);
`
Which is an ideal and will probably involve a few nested functions. At the moment I have the basic Ajax request. Needs to be POST, and doesn't have to handle multiple requests simultaneously.
var lastResponse = '';
function phpFunction(funcName){
var ajaxRequest;
try{
// Opera 8.0+, Firefox, Safari
ajaxRequest = new XMLHttpRequest();
} catch (e){
// Internet Explorer Browsers
try{
ajaxRequest = new ActiveXObject('Msxml2.XMLHTTP');
} catch (e) {
try{
ajaxRequest = new ActiveXObject('Microsoft.XMLHTTP');
} catch (e){
// Something went wrong
alert('Ajax/Browser problem!');
return false;
}
}
}
// Create a function that will receive data sent from the server
ajaxRequest.onreadystatechange = function(){
if(ajaxRequest.readyState == 4){
//Seems to work.
window.lastResponse = ajaxRequest.responseText;
}
}
ajaxRequest.open('POST', 'ajaxCall.php', true);
ajaxRequest.send('func='+funcName); //Need to generate parameters dynamically, not sure how to do that
}
This shouldn't be really complex, but I've never seen a good implementation of it.
Any ideas?