PHP Access-Control-Allow-Origin header not working

Question

I'm working on a JavaScript script that makes AJAX requests to another server (my own domain). The JavaScript script is added to the page using a plugin. It should retrieve data from a PHP file on my domain.

I've added the following header to the PHP file:

<?php
header("Access-Control-Allow-Origin: *");

but the AJAX request still throws an error. Changing the * to the actual domain I plan to use it on doesn't work either. I've searched stack overflow for answers, and came across the following:

// Allow from any origin
if (isset($_SERVER['HTTP_ORIGIN'])) {
    header("Access-Control-Allow-Origin: {$_SERVER['HTTP_ORIGIN']}");
    header('Access-Control-Allow-Credentials: true');
    header('Access-Control-Max-Age: 86400');    // cache for 1 day
}

// Access-Control headers are received during OPTIONS requests
if ($_SERVER['REQUEST_METHOD'] == 'OPTIONS') {

    if (isset($_SERVER['HTTP_ACCESS_CONTROL_REQUEST_METHOD']))
        header("Access-Control-Allow-Methods: GET, POST, OPTIONS");         

    if (isset($_SERVER['HTTP_ACCESS_CONTROL_REQUEST_HEADERS']))
        header("Access-Control-Allow-Headers:        {$_SERVER['HTTP_ACCESS_CONTROL_REQUEST_HEADERS']}");

    exit(0);
}

which doesn't work, either.

I thought my .htaccess file may have somehow affected things, so I deleted it temporarily, but the AJAX request still said no Access-Control-Allow-Origin header was set in the PHP file.

I've found this on daniweb.com: https://www.daniweb.com/web-development/php/threads/461902/php-header-is-not-working-for-access-control-allow-origin which was unresolved.

If the headers have definitely been set (in multiple ways), how come the AJAX request still throws an error, stating the headers haven't been set?

Answer 1

The data that was being sent via the AJAX request was breaking the PHP script - causing it to not execute correctly, which I guess prevented the headers from being set correctly.

Note to all that view this in the future: I assume this is the reason yours may not be working properly too - and most probably why the topics I have viewed prior to writing this were never answered. It is admittedly quite embarrassing. I guess nobody fancied admitting they did something stupid before now.

Make sure the data you send within the request doesn't somehow cause the PHP script to fail to execute.