ftp_get无法连接到文件/目录

I'm connecting to FTP server and trying to download a file from it, but I'm getting an error of

Warning: ftp_get() [function.ftp-get]: Can't open /home/a*******/public_html/files/test.txt: No such file or directory in /home/a*******/public_html/MainHomescreen.php on line 213

if(isset($_REQUEST['download']))    
{
    // connect and login to FTP server
    $ftp_server = "********.*****.***";
    $ftp_username = "a*******";
    $ftp_userpass = "******";

    $ftp_conn = ftp_connect($ftp_server) or die("Could not connect to $ftp_server");
    $login = ftp_login($ftp_conn, $ftp_username, $ftp_userpass);

    if(isset($_POST['checkbox']))
    {
        foreach($_POST['checkbox'] as $selected)
        {
            //echo $selected;

            $local_file = "local.zip";
            $server_file = "/home/a*******/public_html/files/$selected";

            // download server file
            if (ftp_get($ftp_conn, $local_file, $server_file, FTP_BINARY))
            {
                echo "Successfully written to $local_file.";
            }
            else
            {
                echo "Error downloading $server_file.";
            }
        }
    }
    // close connection
    ftp_close($ftp_conn);
}

it's connecting to the server ok as I've checked that but when I try and download a file it comes up with the error above, but does echo the error line too. I'm not sure whether I've connected to it wrong in $local_file and $server_file, which is my best bet as to where I've gone wrong. Hoping someone might be able to help. Thanks.