how do I get information about a photo like the author, the license using PHP?
This information is all available through the Flickr API, if you poke around their docs you may find what you're looking for.
You need to use Flickr's publically avaliable API. Sign up for an API key then have a look at this page (which gives you a basic introduction to contacting the API and parsing serialized PHP. Personally I prefer using XML with SimpleXML).
You may find it easier to use one of the following packages:
Consult the documentation for info on using them.
You call a HTTP REST query, and load the results as a string:
$query = "http://api.flickr.com/services/rest/?method=flickr.photos.getInfo&api_key=" . API_KEY . "&photo_id=" . $photoid . "&format=json&nojsoncallback=1";
data = json_decode(file_get_contents($query));
echo "created by: " . data->photo->owner->username;
echo "link to photopage: " . "http://www.flickr.com/photos/" . data->photo->owner->nsid; . "/" . data->photo->id;
You do this for whatever pieces of data you need from whichever REST calls you require.
All of this is available via the flickr api