Well, "not readable" is usually a permissions problem, but I'm not sure of the "include_php file" syntax.
Usually it would be a link like <a href="http://www.mysite.com/my_search_file.php">my search file</a>.
Also, look at search.tpl and search.php and the way they interact with each other. In search.tpl it "calls" search.php twice, once at the beginning of the file...
<form method="post" action="search.php" name="productsearchform">
and then again towards the end of the file...
<a href="search.php" class="Search">{$lng.lbl_advanced_search}</a>.
Each instance does something different. The first one asks for user input and then processes it. The second one "opens" search.php so the user can choose which input is relevant. Searching x-cart is a pretty involved process.