At least in current versions, /export doesn't return anything over the API, so you can't just output it.
You can use the "file" argument to export to a file, and then read the file though.
e.g.
$exportFileName = 'EXPORT.rsc';
$util = new RouterOS\Util($client = new RouterOS\Client('192.168.88.1', 'admin', 'password'));
$client->sendSync($util->newRequest('export', array('file' => $exportFileName));
sleep(2);//Wait for RouterOS to write the file to disk
$export = $util->fileGetContents($exportFileName);
$util->filePutContents($exportFileName, null);//Optional; Remove the file from the router