hi, this works for me (under linux):
system("./export-questionnaire.php $id_questionnaire >>/dev/null &");
it redirects any output from that script to /dev/null.
but i I encountered trouble when a session was active. obviously, apache sometimes waits for script termination if the external script is started from within a session. in case you encounter this, too, you'll have to destroy the session before this command and restart it afterwards. you can re-register your session vars then.