Wordpress listen to $_POST in functions.php

Question

I currently have the following function in my functions.php which checks if a username exists in the DB.

function check_username() {
       $username = $_POST['user'];
       if ( username_exists( $username ) ) {
           $return['user_exists'] = true; 
       }
       else {
           $return['user_exists'] = false;
       }
       echo json_encode($return);
       die();
}
add_action('wp_ajax_check_username', 'check_username');

I'm using Ajax to call that function with an appropriate username:

$.ajax({
    url : "http://examle.com/wp-admin/admin-ajax.php",
    type : "GET",
    dataType : "json",
    cache : false,
    data : {
        action : 'check_username',
        user: 'test'
    },
    success : function (json) {
        if (json.user_exists) {
            alert(json.user_exists);
        }
    }
});

The problem is that $username returns null instead of test. I've been trying for several hours now different combinations yet they've all returned null.

Assigning this makes the above php function work $username = 'test'; so I'm pretty sure this isn't an issue with the php code.

So my question is, how do I get the variable $username to listen and fetch the correct parameter instead of null?

Answer 1

Please change the type

type : "GET",

to

type : "POST",

Answer 2

While Chinmoy's answer is correct I recommand you to use wp_send_json. When doing ajax this is really useful. It hanldes every essential part of the process including security.

In real life you can use it like that :

wp_send_json_success( $data);//successful requests
wp_send_json_error( $data );// errors

They return an array with $data encoded in json and set some boolean to make your life easier with the jQuery part.