Facebook application requests

Question

i am trying to implement a facebook request in a page tab application. i have seen the tutorial here http://developers.facebook.com/docs/appsonfacebook/tutorial/ but the problem is that i want the code below to be executed only when someone accesses a link like

<a href = "something">Send to friends</a>

any idea about how can this be done? thanks!

   $requests_url = "http://www.facebook.com/dialog/apprequests?app_id=" 
            . $app_id . "&redirect_uri=" . urlencode($the_url_of_the_tab_page)
            . "&message=" . $message;

     if (empty($_REQUEST["request_ids"])) {
        echo("<script> top.location.href='" . $requests_url . "'</script>");
     } else {
        echo "Request Ids: ";
        print_r($_REQUEST["request_ids"]);
     }