Make a WordPress site inaccessible, except via a secret parameter in URL, which then grants access to site for number of days

wp_secret_gateway_only_access

/*
* 
* Make a WordPress site inaccessible except via a secret
* parameter in the URL. Once that URL is visited, 
* grant the visitor access for a number of days via a cookie.
* 
* Gateway URL is of the form: example.com/?{{ secret }}
*/

add_action('init', 'secret_gateway_only_access');
function secret_gateway_only_access() {  

    $secret = "long_alphanumeric_string_here_123_xyz";
    $num_days_of_access = 3;
    $cookie_name = "unique_prefix_access_key";

    if ( isset($_GET[$secret]) ) {
       setcookie($cookie_name, $secret, time()+3600*24*$num_days_of_access);
       wp_redirect(site_url());
    }

    if ( $_COOKIE[$cookie_name] !== $secret ) {
       die(); 
    }
    
}