Suppose you have set-up a webserver (apache2) with PHP enabled and you need to execute some system commands or your own scripts under super user / root priviledges, then the following method may help you to solve the problem, especially under Ubuntu.
Step 1: Run sudo visudo in a Terminal window.
Step 2: Add the below lines at the end of the file, depending on your requirement.
www-data ALL=NOPASSWD: /sbin/iptables, /usr/bin/du
OR
www-data ALL=NOPASSWD: ALL
Step 3: Try an exec command under PHP script to test the settings.
e.g : exec ("sudo iptables -P FORWARD ACCEPT");
