Passing Js Variable To Php

I am trying to pass a JS variable to PHP and have PHP echo the JS variable back. I keep getting an empty null string. What am I doing wrong? function(u){ if(u){

Solution 1:

you should separate php code into a different file and it should be working.

if(u){
    var dt = {'ud':u};
    console.log(dt);

    $.post('xrege.php', dt, function(r){
        console.log(r);
        console.log(typeof r);
    },"json");
}    

name the above code like 123.html and keep the below code in xrege.php

<?php$ud = $_POST['ud'];
echo json_encode($ud);
?>