Basic HTTP Authentication is easily accomplished at the web server level (by way of .htaccess with Apache or inside your nginx configuration file) but did you know you could pull it off inside of a PHP script? You bet your butt you can!

First, you will need to interrogate the _SERVER variables that contain the username and password. If they are set (and are the correct credentials) you can proceed with loading the rest of the page. Otherwise, present the user with the authentication headers with the header function:

if (isset($_SERVER['PHP_AUTH_USER'], $_SERVER['PHP_AUTH_PW'])
    && $_SERVER['PHP_AUTH_USER'] === 'admin'
    && $_SERVER['PHP_AUTH_PW'] === 'password') {

    // User is properly authenticated...

} else {
    header('WWW-Authenticate: Basic realm="Secure Site"');
    header('HTTP/1.0 401 Unauthorized');
    exit('This site requires authentication');
}

Keep in mind that this is just an example and I do not recommend storing plaintext credentials in your script. At worst, you could store the hash for the password and check that against the hash generated from the supplied _SERVER['PHP_AUTH_PW']. At best, you could be loading those credentials from a datastore so that you don’t have any usernames, passwords or hashes in your script. Going that route would also let you easily maintain multiple user accounts instead of just one.