look for the POSTED value in the superglobal variable $_POST[] (or $HTTP_POST_VARS[]). in order to save to hard drive, you're going to have to send the appropriate HEADER() and then ECHO the results.
something like this:
$myURL = $_POST['url_input_name_on_post_form'];
header(foo, bar xyz); // you'll just have to look up header yourself
echo $myURL;