如何通过Ajax获取php返回值？

Im creating a validation form in Ajax and PHP. But i don't have a clue how i should get the value from PHP??

For example:

The validation form is in index.php And the page with the function is checkUser.php. In checkUser i have a global file included with my classes initialized. The checkUser.php look like this:

<?php

$requser = false;
require "core/rules/glb.php";

$user->checkUser($_GET['username']);

The get function comes from the Ajax call i do in the index file. But how do i know that PHP said that the username already exist så that i can make a if statement and paus the script?

Im a beginner, thanks.

And sorry for my english

                                        $.ajax({

                                            type: "GET",
                                            url: "user_add.php",
                                            data: 'username='+$("#jusername").val()+'&email='+$("#jemail").val()+'&password='+$("#jpassword").val()+'&secureSession=23265s"',
                                            success: function()
                                            {
                                                location.href='register.php';
                                            }
                                        });

Jus print out the data, for better help also post the ajax script

<?php

    $requser = false;
    require "core/rules/glb.php";

    print $user->checkUser($_GET['username']);

If you are trying to give a response to the ajax call from php, then you can do it via normal output. Just like

echo json_encode(array("status"=>"FAIL"));
exit();

will send a json response to the ajax call from the php script. like

{"status":"FAIL"}

which you can parse it at the ajax callback and check the status. like

var data = JSON.parse(response);
if(data.status == "FAIL") {
     alert("Ajax call returned failed");
}