calling rsync from php script

Hello,

I have a bash script update.sh that contains:
rsync -- delete -avz -e ssh ${files} [email protected]:/red
when I use that script ./update.sh as root, it works like a charm as I set up the private/public key properly.
When run from a php script through the apache webserver
<?php
exec("update.sh");
?>
The rsync line is ignored. 
I understand that the php script is called under the apache user, when the private/public key is for my root user. How can I set the the keys so that the apache user can run fully the php script? 
More generally, how do php users handles the ssh private/public keys when calling rsync?

Thank you