Token for prevention of CSRF doesn’t work on live server

I wrote a script which generates an authentication token in order to prevent CSRF attacks. It works well on local server but returns the 403 error on live server

Here is the code that checks if a token already exists or not

    // Generate authentication token to prevent CRSF attacks
// Check if a token is present for the current session
if (!isset($_SESSION["auth_token"])) {
    // No token present, generate a new one
    $auth_token = bin2hex(random_bytes(35));
    $_SESSION["auth_token"] = $auth_token;
} else {
    // Reuse the token
    $auth_token = $_SESSION["auth_token"];
}

And here is the code that validates the token upon submission

if ($_SERVER["REQUEST_METHOD"] == "POST") {
// Validate token to avoid CSRF attacks
$token = trim($_POST['auth_token']);
if (!isset($token) || !isset($_SESSION['auth_token']) || $token !== $auth_token) {
    // show an error message
    echo '<h1 class="error">Error: invalid form submission</h1><p>Your request was denied as this request could not be verified.</p>';
    // return 403 http status code
    http_response_code(403);
    die();
    exit();
}
}

UPDATE

I checked in on my error_log.php and this was the error displayed

session_start(): Cannot start session when headers already sent in /home/refermec/public_html/user/login.php on line 15

I have the same code as stated earlier in all pages with a form that requires authentication

Answer

According to your error, you have content ABOVE session_start(); Once any content, no matter a HTML comment, an echo, a header() happens before session_start();, php will throw that error.

All these things need to come AFTER the session_start(); …

Make sure session_start(); is at the top of the file, or at the top of an included file.

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Answer