su idn't working from within php

hi all i was trying to execute some commands as an authorized user from within my php script with exec ("su <username> -c \"sudo <command>\" 2>&1;", $output); but it gives me standard in must be a tty how can i make it work? it runs perfectly at command prompt I even tried to ssh, but the problem is where should i copy the rsa-key generated with keygen, as the scripts are run as apache/http user.......... Plz help me out of this Thanx in advance Nitin