AJAX内的PHP代码以发布

Question

I'm trying to echo PHP variable $last_id to AJAX to pass it onto PHP $_POST. May be I can skip AJAX all together and just transfer variable inside $_POST. Do I have to use echo json_encode?

Here is my code to see what I'm trying to accomplish here (any help appreciated):

registration.php

<?php
if(isset($_POST['submit'])) {
require 'connect.php';

$sql = "INSERT INTO table (username, password) VALUES ('".$_POST["username"]."','".$_POST["password"]."')";

if (mysqli_query($con, $sql)) {
$last_id = mysqli_insert_id($con);
    echo $last_id;
?>

myScript.js

// ----AJAX Post to PHP----->
function ajax_post(ele,div){
// XMLHttpRequest object
var hr = new XMLHttpRequest();

// variables send to PHP file
var url = "insert.php";
var userId = "<?php echo $last_id; ?>";
var vars = "userId="+userId;
hr.open("POST", url, true);
hr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
hr.onreadystatechange = function() {
    if(hr.readyState == 4 && hr.status == 200) {
        var return_data = hr.responseText;
        document.getElementById("status").innerHTML = return_data;
        }
    }
}

insert.php

<?php
require 'connect.php';

$userId = $_POST['userId'];

$sql = "INSERT INTO table2 (userId) 
VALUES ('$userId')";
?>

Answer 1

Hi this is how you can pass php variable into external js file,

<?php
$last_id= "Red";
?>
<script type="text/javascript">var color = "<?= $last_id?>";</script>
<script type="text/javascript" src="myscript.js"></script>

Try and let me know