Thanks!
Finally I found the solution:
In search_function.php, add the following code right after the function statement:
ob_start();
require("my_include.php");
$my_include = ob_get_contents();
ob_end_clean();
then, add my_include to array:
$t_mstrings = compact('my_include','powered_by_link','title_message','phpdig_version','re sult_message' ,'nav_bar','ignore_message','ignore_commess','pages_bar','previous_link','n ext_link','templates_links');
and insert <phpdig:my_include/> in the template file ... that's all.
Thanks a lot for your time Charter!