Unable to get output from exec "ssh remote-server 'ping -c4 ip'"

Hi, I need to develop an interface that will, for example, ping IPs from a remote server. So the php script will be hosted in a server with ssh access (key) to the remote server. I can make it work from the console but when I try $cmd = "ssh remote-server 'ping -c 4 ip'" exec($cmd, $ouput); the output is empty. the same command (with the remote-server and IP changed to the real ones) works fine from command line. I've added the key/config in the webserver's user .ssh directory so I am assuming that that there is something wrong with my setup. any ideas?