如何让www-data执行ping？

Question

I try to execute a ping command with the user www-data

$command    = 'ping -c 4 www.stackoverflow.com 2>&1';
$result     = shell_exec($command);

But i always get ping: icmp open socket: Operation not permitted. So i tried to allow the command by executing visudo and adding this line:

www-data ALL = NOPASSWD: /bin/ping

Then i restarted apache2 and tried it again, but i still get Operation not permitted.

How can i solve this?