how to execute a remote command from php

Hello, Since last week I am trying to execute a remote machine command from php without success. Since ssh ask for a password, I managed to use ssh with dsa authentication so it's possible to call the remote command without password and only with one line: ssh -l victor 192.168.129.211 -i /root/.ssh/id_dsa "whoami". It works perfectly from command line but I have nothing if I try to do the same from php: exec("ssh -l victor 192.168.129.211 -i /root/.ssh/id_dsa \"whoami\"",$results); $results is empty after the execution. I wonder if somebody could answer this question. I have found no solution googling it. Thank you. Regards, Victor.