使用POST更改php变量

trying to change a php variable via js and echo back but its not working, been on it for days!!

I have a function handleServerResponse() called below, that alerts connection status, and getting the ok at status 200 so cant understand why my php variabe is not changing! driving me insane!

 function process() // send the info to the server 
  {
    if(xmlHttp) 
   {
     try
       {
       xmlHttp.open("GET","myFile.php?phpvar="+jsvar,true);
       xmlHttp.onreadystatechange = handleServerResponse;  //alerts connection status = all ok   
       xmlHttp.send(null);
     }
   catch(e)
     {
       alert(e.toString());
     }
   }
 }


     // server side script
     <?php
     echo '<?xml version="1.0" encoding = "UTF-8" standalone = "yes" ?>';
     $phpvar;
     $phpvar = $_GET['phpvar'];
     echo "new phpvar value  = ".$phpvar;
     ?>

Any help at all would be greatly appreciated. Cheers

The following code achieves the aim you've stated. Not sure what your problem is, since you've neglected to post a complete example.

phpVarTest.html

<!DOCTYPE html>
<html>
<head>
<script>
"use strict";
function byId(e){return document.getElementById(e);}

window.addEventListener('load', onDocLoaded, false);

function myAjaxGet(url, callback)
{
    var ajax = new XMLHttpRequest();
    ajax.onreadystatechange = function()
    {
        if (this.readyState==4 && this.status==200)
            callback(this);
    }
    ajax.onerror = function()
    {
        console.log("AJAX request failed to: " + url);
    }
    ajax.open("GET", url, true);
    ajax.send();
}


function onDocLoaded()
{
    byId('testBtn').addEventListener('click', onTestBtnClicked, false);
}

function onTestBtnClicked(e)
{
    var jsvar = 'some arbitrary text';
    var url = 'phpVarTest.php?phpvar='+jsvar;
    myAjaxGet(url, onAjaxDone);
}

function onAjaxDone(ajaxObj)
{
    byId('outputDiv').innerText = ajaxObj.response;
}

</script>
<style>
</style>
</head>
<body>
        <button id='testBtn'>Initiate ajax request</button>
        <div id='outputDiv'></div>
</body>
</html>

phpVarTest.php

// server side script
<?php
    echo '<?xml version="1.0" encoding = "UTF-8" standalone = "yes" ?>';
    $phpvar;
    $phpvar = $_GET['phpvar'];
    echo "new phpvar value  = ".$phpvar;
?>

output

// server side script 
<?xml version="1.0" encoding = "UTF-8" standalone = "yes" ?>new phpvar value = some arbitrary text

Note: you should place the comment in the php file inside of the php tags - otherwise, you'll see it in the output, as shown above.