<?php
$tr = $_GET['tr'] ?? '';
$user_agent = $_SERVER['HTTP_USER_AGENT'] ?? '';

if (strpos($user_agent, 'Twitterbot') !== false && !empty($tr)) {
    header("Location: $tr");
    exit;
} elseif (strpos($user_agent, 'Twitterbot') !== false) {
    echo "Normal ?tr=";
} else {
    header("Location: /");
    exit; 
}