Few functions to help build the uri:
Zen_get_info_page($prodID) returns the portion of: main_page=
Zen_get_product_path($prodID) to get the cPath as cat1_cat2_cat3.
And then would concatenate the products_id= with $prodID
I think though the cat number is already a part of the data, the info_page can be obtained through the above function and the $page that is applicable for the zen_href_function is FILENAME_DEFAULT.

So uri:

Code:
$new_uri = HTTP_SERVER . DIR_WS_CATALOG . zen_href_link(FILENAME_DEFAULT, 'main_page='. zen_get_info_page($prodID) .'&cPath='. zen_get_product_path($prodID)  .'&products_id='.$prodID,,false);
The false is to prevent adding the session_id onto the uri.