Overriding Template Files by using hook_theme()

You will find several other template files as you browse through the module and theme directories of
your site. For example, the custom search module uses custom_search.tpl.php for rendering custom search. The
custom search module creates a number of variables and exposes those variables to the custom_search.tpl.php file.
The designation of the custom_search.tpl.php file as the template file for comments is made through a call to hook_theme() by passing 'template' => 'custom_search' as one of the values in the array. There’s no need to specify the custom_search.tpl.php file extension as Drupal assumes that’s what you mean.
I’ll cover additional details on how to create and expose variables to your template in a bit.

/**
* Implements hook_theme().
*/
function custom_search_theme() {
return array(
'custom_search.tpl.php' => array(
'render element' => 'form',
'template' => 'custom_search-nameform',
),
);