在远程服务器上执行命令

Question

我有星号服务器所以我想通过PHP脚本远程执行命令，我试过一个：

<?php

$server = "192.168.1.8";


//specify your username
$username = "root";

//select port to use for SSH
$port = "22";

//command that will be run on server B
$command = "ls -l";

//form full command with ssh and command, you will need to use links above for auto authentication help
$cmd_string = "ssh -p ".$port." ".$username."@".$server." ".$command;

//this will run the above command on server A (localhost of the php file)
exec($cmd_string, $output);

//return the output to the browser
//This will output the uptime for server B on page on server A
echo '<pre>';
print_r($output);
echo '</pre>'; ?>

但没有任何反应，问题是什么？