Mobile Theme Switcher

mobile.php

<?php

/**
 *
 * Mobile theme switcher code - pulled from Mobile theme switch
 * Original URI: http://wordpress.org/extend/plugins/mobile-theme-switcher/
 * Original Author: Jonas Vorwerk, http://www.jonasvorwerk.com/
 * 
 *
 * Bypass this check in any browser by using http://myurl/?mobile=on or http://myurl/?mobile=off This is locally being stored using a cookie. 
 * 
 * param $themes - set to genesis (parent theme)
 * param $style - set to child theme directory name (child theme)
 * 
 */
if( is_admin() )
	return;

session_start();

if( !isset( $_SESSION['mobilethemeswitch'] ) )
	$_SESSION['mobilethemeswitch'] = '';
if( isset( $_GET['mobile'] ) && $_GET['mobile'] == "off"){
	$_SESSION['mobilethemeswitch'] = "off"; 
} else if( isset( $_GET['mobile'] ) && $_GET['mobile'] == "on"){
	$_SESSION['mobilethemeswitch'] = "on";
}

//mobile browsers - this list can be expanded as needed
$iphone = strpos($_SERVER['HTTP_USER_AGENT'],"iPhone");
$ipod = strpos($_SERVER['HTTP_USER_AGENT'],"iPod");
$android = strpos($_SERVER['HTTP_USER_AGENT'],"Android");
$palmpre = strpos($_SERVER['HTTP_USER_AGENT'],"webOS");
$berry = strpos($_SERVER['HTTP_USER_AGENT'],"BlackBerry");
$iemobile = ( strpos($_SERVER['HTTP_USER_AGENT'],"iemobile") || strpos($_SERVER['HTTP_USER_AGENT'],"IEMobile") );

if ( ((($iphone || $android || $palmpre || $ipod || $berry !== FALSE || $iemobile) === true) || $_SESSION['mobilethemeswitch'] == "on") && $_SESSION['mobilethemeswitch'] != "off" ) { 
	add_filter('stylesheet', 'getThemeStyle');
	add_filter('template', 'getTemplateStyle');
} 

function getTemplateStyle(){
    $themes = 'genesis';
    return $themes;
}

function getThemeStyle(){
    $style = 'dcsi-mobile';
    return $style;
}

Love this!

(1) So $themes is the default theme and $style is the mobile theme?

(2) Would this code go in index.php or wp-config.php in my root?