Somewhere in the scripts should have a SELECT that determine the item who will be displayed. I need to "hack" this script to add a condition that selects the item if the contents of a extra field match a variable I set in the index.php (template). In other words...

Imagine a variable $show = Y living in the index.php script and an extra field named "condpublish". Where (and how) can I put a "WHERE" clause to check if "condpublish" = $show and display or not the item according "condpublish" value?

(The "published" field is not enough to my objective that is show an item according the TEMPLATE in use).