вернуть json 200 ok и json 500 error на ajax

Помогите решить задачу вот js

$("#registerform").on( "submit", function( event ) {
event.preventDefault();
var formData = new FormData(document.forms.editForm);
formData.append("userData", true);
        $.ajax({
            url: '/api/user/update.php',
            contentType: false,
            processData: false,
            type: 'POST',
            data: formData,
            success: function(json_response){
                console.log(json_response);
            },
            error: function(){
                alert('Error!');
            }
        });
});

update.php

  if (isset($_POST['userData'])){
 $id = (int)$_POST['id'];
 $full_name = mysqli_real_escape_string($con, $_POST['full_name']);
 $email = mysqli_real_escape_string($con, $_POST['email']);
 $phone = mysqli_real_escape_string($con, $_POST['phone']);

require_once("connection.php");
global $con;
session_start();
$response = array();
if (checkEmail() && checkSID()) {
    $query = mysqli_query($con, "UPDATE `usertbl` 
                       SET full_name='$full_name', email='$email', phone='$phone' 
                       WHERE id='$id'
                       ");
    if (mysqli_affected_rows($con)){
        /**
         * JSON 200
         */
        echo "if";

    } else {
        /**
         * JSON 406 and 500
         */
        echo "else";

    }
}