Why is my CORS Request Failing with \'Origin is not allowed by Access-Control-Allow-Origin\'?

CORS Not Functioning in PHP

Problem Description:

When attempting to transmit form data via CORS (Cross-Origin Resource Sharing) from www.siteone.com to www.sitetwo.com, the following error is encountered:

XMLHttpRequest cannot load http://www.sitetwo.com/cors.php. Origin http://www.siteone.com is not allowed by Access-Control-Allow-Origin.

Despite setting the following headers in cors.php on www.sitetwo.com:

header('Access-Control-Allow-Origin: *');
header('Access-Control-Allow-Methods: POST, GET, OPTIONS');

Solution:

The problem lies in the way CORS request headers are handled. The following updated code for cors.php provides a more comprehensive response to CORS requests:

<code class="php"><?php
// 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);
}

// Respond to the request
echo "You have CORS!";
?></code>

This revised code allows cross-origin requests from any origin, validates request methods and headers, caches the CORS response for a day, and responds appropriately to OPTIONS requests.

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