$messages is an array of objects, you can't output it as a string.

I would create an export function and export it as an array.

function export_message($message) {

    return [
        'guid' => $message->guid,
        'message' => $message->description,
        'toId' => $message->toId,
        'time_created' => $message->time_created,
        // whatever other info you might need
    ];

}

 

Then you can use that in your service

$messages = elgg_get_entities_from_metadata(...);
$exported = [];
foreach ($messages as $m) {
    $exported[] = export_message($m);
}

return $exported;

Thank you very much Matt.

Your reply was very helpful

Kind Regards

Alberto