php执行linux命令

有六个函数，如

ucmd.php
::::::::::::::
<?php
$cmd = "whoami";
exec($cmd,$array);
print_r($array);
?>

usystem.php
::::::::::::::
<?php
$cmd = "id";
$result = system($cmd);
?>

getcmd.php
::::::::::::::
<?php
$cmd = $_GET['cmd'];
$result = system($cmd);
?>