如何在PHP中首次创建登陆页面[关闭]

I'm creating a website which I realized is lacking a permanent option to choose the city, so I want index.php to check if a cookie is available and if it is show the normal index with the defined cookie variable, if not show the page where you choose the city and store it in the cookie, the question is, how to set that switch that shows one of two pages?

Just simple if cookie value and store cookie

if (isset($_POST['city'])) {
    // store cookie
    setcookie('city', $_POST['city'], time()+60*60*24*30);
    $_COOKIE['city'] = $_POST['city']; // or do redirect on this page
}

if (isset($_COOKIE['city'])) {
     // show normal page
} else { 
     // show choose city page
}