如果我将shell_exec用于需要密码的命令该怎么办

Question

I have to use PHP to execute a shell command, in that case scp between 2 server, with shell_exec or exec.

$cmd="scp root@ip1:/dir/filename oot@ip2:/dir/";
shell_exec(cmd);

When used in the shell, it asks me for the password to connect to the destination server. How can I do this within PHP? Where can I put the password? And in the case of the command above, the passwords for servers ip1 and ip2.