Inject Link Using Rules->CustomPHP Short version: I'd like to change what is displayed in a certain rendered Node field, if the viewing user meets certain conditions. I think the Rules action "Execute Custom PHP" can do this? But I have no experience with PHP.

Less short version: On the site, users read artistic content. A link at the top of the page gives them the option to buy said content. This should give them access to a .zip file that contains Goodies.

The node's Download field shows a link to go and buy the content. On page load, if the user owns the content (determined by a Rules condition), I want the link to be replaced with a download link. Ideally, the new link would be retrieved from a hidden field on the node.

Clever use of the "hidden" parameter could also achieve this effect, but I imagine the solution would look pretty similar. Or is this at all doable in the way I've described?

Pointers to solutions or alternatives would both be appreciated. Thanks!
If anyone's interested in what I did here: this is best done using Views and Panels. Determining if a user owned a certain item required Views PHP. Once I got that working, it was pretty extensible, and I was able to implement some other displays based on it.