WordPress Search Queries - wordpress

Question

I have added within my WordPress 3.1 site, the following code at the bottom of my sidebar.php file:
<div id="search_box">
<form id="searchform" action="http://www.service.com/" method="get" role="search">
<input id="s" type="text" name="s" class="search" value="Search site" size="19" maxlength="80" id="white_box" onfocus="if (this.value=='Search site') this.value = ''"/>
<input id="searchsubmit" type="image" class="submit" value="submit" src="<?php bloginfo( 'template_url' ); ?>/images/search_btn.jpg" />
</form>
</div>
As I have coded this search process myself, when I place a some text within my search text box and press the "Search" button, it looks as if, is is calling the page search.php within my theme and displaying the results.
Questions:
1) where/how does it know when I press the "Search" button to go off and call search.php?
2) if possible only, how can I change it to call a different php file instead of the search.php
Thanks.

Answer 1

Use template filter
add_filter( 'template_include', 'template_include', 10 );
and change the template as
function template_include($template)
{
if(your condition here){
$template = get_template_directory().'/your-template.php';
}
return $template;
}

Answer 2

-1. All requests for a WP site go to a single page that routes them based upon specific criteria. Thus when a search is performed it knows that it is a search and directs to the search.php page.
Specifically it goes to index.php that loads wp-blog-header.php.
-2: Everything you need to know should be right here: http://codex.wordpress.org/Creating_a_Search_Page#Using_the_page.php

Answer 3

Wordpress assumes the request is a search if it sees an 's' GET variable.
You can hook into the template_redirect action (in your theme's functions.php file) and load a different template like so:
add_action('template_redirect', 'my_template_select');
function my_template_select() {
if (is_search()) {
load_template(TEMPLATEPATH . '/foobar.php');
exit;
}
}