Can I restart iTunes remotely from my iPhone/iPad?
One option would be to run sudo visudo to edit the sudoers file and add the following line:

apacheuser ALL=(youruser) NOPASSWD: /usr/bin/osascript


Be sure to change apacheuser to the account that apache runs under and youruser to the account that iTunes runs under. This allows apache to run AppleScript commands as the chosen user without prompting for a password. You can find more information about this in the sudoers man page.

However, this could introduce a security hole, since the apache user would then be able to perform any action as the user that AppleScript allows. A better option would be to create an intermediary shell script that takes start and stop as an argument. 

#!/bin/sh

case $1 in
    stop)  /usr/bin/osascript -e 'tell application "iTunes" to quit';;
    start) /usr/bin/open /Applications/iTunes.app;;
    *)     echo 'Please use "start" or "stop" as an argument.';;
esac


Save this in a file somewhere, and make it executable with chmod +x. In this case, your line in the sudoers file would now be:

apacheuser ALL=(youruser) NOPASSWD: /Some/Directory/iTunesControl.sh


Then, from your PHP script, call the shell script with:

sudo -u youruser /Some/Directory/iTunesControl.sh stop|start