login authentcation via browser prompt using PHP

<?php
if (($_SERVER['PHP_AUTH_USER'] != 'a') || ($_SERVER['PHP_AUTH_PW'] != 'a')) {
      header('WWW-Authenticate: Basic Realm="Secret Stash"');
      header('HTTP/1.0 401 Unauthorized');
      print('You must provide the proper credentials!');
      exit;
   }
else{
echo 'login done';
//do stuff here
}
?>