Finding parts in the url with php

1

I have this url,
http://www.martystudio.com/index.php?error=101

i want to use php to lok at the error part of the url, if its found, look at what it equals. if it finds 101 $sendto = home if its 022 $sendto = send.php, if it dosent find the error , $sendto = home.php. here is the format that is somthing like what im trying to use

<?php
if (is_null($_POST[‘error’]) == TRUE) {
$sendto = “home.php”;
}
?>