php basic auth

<?php

if (!isset($_SERVER["PHP_AUTH_USER"]) || !isset($_SERVER["PHP_AUTH_PW"])) {
    http_response_code(401);
    header("WWW-Authenticate: Basic realm=\"?\"");
    exit();
}

if ($_SERVER["PHP_AUTH_USER"] != 'a' || $_SERVER["PHP_AUTH_PW"] != 'a') {
    http_response_code(401);
    exit();
}

exit("hey");