ftp_exec error Warning: ftp_exec() [function.ftp-exec]: 'SITE EXEC ls -la > file.txt':

<?php
// variable initialization
$command = 'ls -la > file.txt';

// set up basic connection
$conn_id = ftp_connect('192.168.1.2');

// login with username and password
$login_result = ftp_login($conn_id, 'root', '123456');

// execute command
if (ftp_exec($conn_id, $command)) {
echo "$command executed successfully\n";
} else {
echo "could not execute $command\n";
}

// close the connection
ftp_close($conn_id);
?>