What are the differences between Response Headers and Request Headers in PHP when dealing with Basic Authentication?

When dealing with Basic Authentication in PHP, it is important to understand the differences between Response Headers and Request Headers. Request Headers are sent from the client to the server to provide information about the request being made, while Response Headers are sent from the server to the client to provide information about the response. When implementing Basic Authentication in PHP, the server will typically send a 401 Unauthorized response along with a WWW-Authenticate header in the Response Headers to prompt the client to provide credentials in the Request Headers.

<?php

// Check if the Authorization header is present in the Request Headers
if (!isset($_SERVER['PHP_AUTH_USER'])) {
    header('WWW-Authenticate: Basic realm="My Realm"');
    header('HTTP/1.0 401 Unauthorized');
    echo 'Unauthorized access';
    exit;
} else {
    // Validate the credentials provided by the client
    $username = $_SERVER['PHP_AUTH_USER'];
    $password = $_SERVER['PHP_AUTH_PW'];

    // Your authentication logic here
}

// Proceed with the rest of your PHP code