Simple login

login.php

<?php

if($_SERVER['REQUEST_METHOD'] !== 'POST') {
    http_response_code(405); // Method Not Allowed
    echo 'Data should be POST\'ed';
    die();
}

if(!isset($_POST['username'])) {
    http_response_code(400); // Bad Request
    echo 'Username not specified';
    die();
}

if(!isset($_POST['password'])) {
    http_response_code(400); // Bad Request
    echo 'Password not specified';
    die();
}

if($_POST['username'] === 'aap' && $_POST['password'] === 'aap') {
    http_response_code(200); // OK
    echo 'Success!';
    die();
}

http_response_code(401); // Unauthorized
echo 'Invalid username or password';