AJAX，PHP和多个变量（不是JQuery）

I have a PHP file that has some radio inputs on them which I wish to use to create a two variables (text string and a integer) via an AJAX (Not JQuery) call which I can use in the PHP file. I can work with a single variable which out puts to "getElementById" but as soon as I add a second, the code fails. 1/ How do I get more than one variable back from the AJAX? 2/ I can work with "getElementById" for collecting the new PHP variables but as I just want to set a couple of PHP variables for use later, I'm not sure this is the best method of collecting the response.

After reading around the web and Stackoverflow, I have got as far as the code below but no success.

<script>
function showUser(str)
{
if (str=="")
{
document.getElementById("Div1").innerHTML="";
document.getElementById("Div2").innerHTML="";
return;
} 

if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp=new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
{
if (xmlhttp.readyState==4 && xmlhttp.status==200)
{
var response = (ajaxRequest.responseText).split(';',2);
var intVariable = response[0];
var StringVariable = response[1];
document.getElementById("Div1").innerHTML=xmlhttp.intVariable;
document.getElementById("Div2").innerHTML=xmlhttp.StringVariable;   
}
}
xmlhttp.open("GET","ajax_info.php?bookingstate="+str,true);
xmlhttp.send();
}
</script>


<form name='users'>
<radiogroup>
<input type="radio" name="user" value="1" onclick="showUser(this.value)" <?php if         ($select_state == '1') echo 'checked="checked"'; ?> >1
    <br>
    <input type="radio" name="user" value="2" onclick="showUser(this.value)" <?php if ($select_state == '2') echo 'checked="checked"'; ?> >2
<br>
<input type="radio" name="user" value="3" onclick="showUser(this.value)" <?php if     ($select_state == '3') echo 'checked="checked"'; ?> >3
<br>
</radiogroup>
</form>

And my ajax_info.php file

<?php

if ($bookingstate == "1") {
$select_state_name = "Booking state: 1";
$select_state1 = "1";


} elseif ($bookingstate =="2") {
$select_state_name = "Booking state: 2";
$select_state1 = "2";

} elseif ($bookingstate == "3") {
$select_state_name = "Booking state: 3";
$select_state1 = "3";

}

echo '$select_state1;$select_state_name;';

?>