<form method="POST">
    <input type="url" name="url" />
    <input type="submit" value="Check" />
</form>
<?php
    $source = file_get_contents ('http://www.downforeveryoneorjustme.com/' . urlencode ($_POST['url']));
    if (strstr($source, 'looks down'))
    {
        echo 'Jup, hijs down.';
    }
    else if (strstr($source, 'interwho'))
    {
        echo 'Ongeldig';
    }
    else
    {
        echo 'Nee hoor, hijs up.';
    }
?>