Don't want to use an intermediate file? Use 'php://output' as the filename and then capture the output using output buffering.
ob_start();
$result = ftp_get($ftp, "php://output", $file, FTP_BINARY);
$data = ob_get_contents();
ob_end_clean();
Don't forget to check $result to make sure there wasn't an error. After that, manipulate the $data variable however you want.