you can use like this situation:
for example, you have a page: http://www.example.com/page.php
then in that page.php, insert this code:
if (!empty($_GET['doaction']) && $_GET['doaction'] == blabla ){
echo '<script>alert("hello");</script>';
}
then, whenever you visit this url: http://www.example.com/page.php?doaction=blabla
then the alert will be automatically called.