<?php
//...
$tags = $api->photos_search(array(
'text' => 'stuff',
));
foreach ($tags['photo'] as $photo) {
// want to get get the user's actual username from the the "owner"
$ownerId = $photo['owner'];
// getUsernameById($ownerId); // <-- how do you do this?
}
I have never used that library, but according to the Flickr API, the function is called originally flickr.people.getInfo
.
That PHP class follows this naming convention. I would try
$userinfo = $api->people_getInfo($ownerId);
var_dump($userinfo['username']);
Good luck!