无法使用PHP运行Ajax

I'm new to php and ajax. I want to run simple code of ajax with php but the code is not working as intended. I want load some text in the page when onchange happen

source code ajax.php

<select id='mySelect' onChange="MyFunction()">
    <option value='option1'>Option1</option>
    <option value='option2'>Option2</option>
    <option value='option3'>Option3</option>
</select>




<script>

function MyFunction()
{
    var params="myName=2";
    var xmlHttp;
    if(window.XMLHttpRequest)
        xmlHttp=new XMLHttpRequest();
    else
        xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
    xmlHttp.open("GET","edittest.php",true);
    if(xmlHttp)
    {

    xmlHttp.onreadystatechange=function(){
        if(xmlHttp.readyState==4 && xmlHttp.status==200)
        {

            alert(document.getElementById('mySelect').value);
            }
        }
    }

    xmlHttp.send(params);
    }

</script>

edittest.php source code

<?php

if(isset($_GET))
{
    for($i=0;$i<1000;$i++)
    {
        echo($_GET['myName']);
        }
    }

?>

For a much simpler experience, I suggest you use jQuery AJAX. Here is the code. Check the Documentation for more details.

function MyFunction()
{
    var myName=2;
$.ajax({
    url: "/edittest.php",
    data: {
        myName: myName
    },
    type: "GET",
    success: function(response){
        console.log(response);
    }
});
}

Your problem is that you declare var params="myName=2";, but you never use it. Your script works, but since it is just requesting the edittest.php, it wont show you anything, since your PHP script only prints out the myName variable.

You should add the parameters at the end of your request like this:

<select id='mySelect' onChange="MyFunction()">
    <option value='option1'>Option1</option>
    <option value='option2'>Option2</option>
    <option value='option3'>Option3</option>
</select>

<script>
    function MyFunction() {
        var params = "myName=2";
        var xmlHttp;
        if (window.XMLHttpRequest)
            xmlHttp = new XMLHttpRequest();
        else
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");

        xmlHttp.open("GET", "edittest.php?" + params, true);
        if (xmlHttp) {
            xmlHttp.onreadystatechange = function () {
                if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                    alert(document.getElementById('mySelect').value);
                }
            }
        }
        xmlHttp.send(params);
    }
</script>

(Notice the xmlHttp.open("GET", "edittest.php?" + params, true); part.)