ab_redirector.php

<?php

$config_file = "default.json";
$config = json_decode(file_get_contents($config_file));

$host = $config["default_host"];
$uri = $_SERVER["QUERY_STRING"];
if ($config["ab_enabled"]) {
    if (in_array($_COOKIE["ab"], $config["ab_hosts"])) {
        $host = $_COOKIE["ab"];
    } else {
        $host = array_rand($config["ab_hosts"]);
        setcookie("ab", $host, 86400);
    }
}

header("Location: http://$host$uri");