PHP code for detecting a correct Discord server from a custom invite link, with a fallback to a default invite link

discord.php

<?php

/**************************************************************
 * PHP Script for Discord server checking
 * Author: Marek Poláček
 * License: MIT License
 * File: discord.php
 * Description: Redirects to the Discord server invite link.
 **************************************************************
 * How to use:
 * 1.   Get your server IDs and place them inside the
 *      $discordServer variable
 * 2.   Get your custom invite codes (or prepare some beforehand)
 *      and place them in the $customInviteCode variable
 * 3.   Get your fallback invite codes (your default ones)
 *      and place them in the $fallbackInviteCode variable
 * 4.   Use the comments as a guide to how to set things up.
 * 5.   Upload the file to your PHP server, optionally use
 *      .htaccess for a nice looking URL rewrite for each
 *      Discord server you own.
 **************************************************************/

// Check if the 'server' query parameter is set to determine which Discord server to redirect to
$devMode = isset($_GET['server']) && $_GET['server'] === 'dev';     // You can remove that or customize and/or add more checks
                                                                    // for your other Discord servers. Anything else will just
                                                                    // lead to your main Discord server.

// Set the Discord server ID based on the server type
// Customize that according to your checks
// To copy the server ID, you need to enable Development Mode in your Discord account.
// Then right click the server you want to copy the ID of, and select Copy Server ID (at the bottom of the menu).
if ($devMode) {
    $discordServer = 'DEV-DISCORD-ID';      // Development server ID    => Replace with your actual server ID
} else {
    $discordServer = 'MAIN-DISCORD-ID';     // Main server ID           => Replace with your actual server ID
}

// Custom invite code for the Discord server (optional)
$customInviteCode = ($devMode) ? 'devserver' : 'mainserver';    // Replace with your custom invite codes if you have them set up,
                                                                // otherwise these will be ignored and the fallback invite codes
                                                                // will be used instead

// Fallback to the default invite link if the custom invite code is not set
$fallbackInviteCode = ($devMode) ? 'devinvite' : 'maininvite';  // Use previously generated codes as fallback => Replace with actual
                                                                // fallback invite codes (will look something like this: "aXTd45p")
                                                                // DO NOT DELETE YOUR FALLBACK INVITE CODES!!! If you do delete them,
                                                                // you'll be left with a disfunctional invite link from your website.
                                                                // It's best practice to leave your default invite codes existing
                                                                // To make sure you have a safe fallback in case your custom invite
                                                                // code stops working for the reason your server dips below Level 3
                                                                // Nitro boosts, in which case your server invite will cease to exist
                                                                // and is vulnerable for custom invite code sniping (could be used to
                                                                // guide your users to a malicious Discord server impersonating you).

// Construct the invite URL using the custom invite code and perform a quick check to ensure it's valid and points to the correct server
$inviteUrl = "https://discord.gg/{$customInviteCode}";
$headers = @get_headers($inviteUrl);
if ($headers && strpos($headers[0], '200') !== false) {
    // If the custom invite code is valid, check if it points to the correct server by fetching the invite details
    $inviteDetailsUrl = "https://discord.com/api/v10/invites/{$customInviteCode}?with_counts=true";
    $inviteDetails = @file_get_contents($inviteDetailsUrl);
    if ($inviteDetails) {
        $inviteData = json_decode($inviteDetails, true);
        if (isset($inviteData['guild']) && $inviteData['guild']['id'] === $discordServer) {
            // If the invite points to the correct server, use it
            header("Location: {$inviteUrl}");
            exit;
        }
    }
}

// If the custom invite code is not valid or does not point to the correct server, redirect to the fallback invite link
// MAKE SURE THESE INVITE CODES ARE EXISTING AND POINT TO YOUR SERVER, YOU DON'T WANT TO EXPOSE YOUR USERS TO A MALICIOUS DISCORD SERVER
// OR PROVIDE AN INVALID INVITE LINK
$fallbackInviteUrl = "https://discord.gg/{$fallbackInviteCode}";
header("Location: {$fallbackInviteUrl}");
exit;