Registration form AJAX check for existing username (simple version)

Question

You’re POSTing user_name but in your code you’re checking for register_name, you should instead check $_POST['user_name'] instead.

You should also be checking to make sure a value is actually being passed as well, this is how I would do it instead (setting unavailable as default):

function mpl_check_username() {

    $response = array(
        'status' => 'unavailable',
        'text' => __( 'Username unavailable' )
    );

    $username = array_key_exists( 'user_name', $_POST ) ? sanitize_text_field( $_POST['user_name'] ) : false;

    if ( $username && ! username_exists( $username ) ) {
        $response['status'] = 'available';
        $response['text']   = __( 'Username available' );
    }

    echo json_encode( $response );
    die();
}

All the username_exists function does is this code below:

$user = get_user_by( 'login', $username );
if ( $user ) {
    $user_id = $user->ID;
} else {
    $user_id = false;
}

You could also try this in your code to remove the possibility of the username_exists filter being used to return true for some weird reason.