I need help making this logic more efficient - php

Question

EDIT - I have completely re-written the logic below but I'm sure it can still be improved.
EDIT - The current working code for the alternate theme selector is at GitHub. I'd still like to improve on it but it's a lot cleaner than what I started with. At this point, I'll take the issue thread there since user testing will reveal the next steps.
New Logic:
<?php
// Fetch theme options array
global $ats_plugin;
global $is_IE;
// Initialize the main variables
$ats_active = 0;
$the_theme = null;
// We only want to run this fat chunk of logic on IE since it's IE with the issues
if ( $is_IE ) {
// Is a supported useragent active?
function checkActive($data) {
global $ats_active;
$user_agent = $_SERVER['HTTP_USER_AGENT'];
switch($user_agent) {
case strpos($user_agent, 'MSIE 5') :
if ($data['ie5_switch'] == 1 && !empty($data['ie5_theme'])) {
$ats_active = 'ie5';
};
break;
case strpos($user_agent, 'MSIE 6') :
if ($data['ie6_switch'] == 1 && !empty($data['ie6_theme'])) {
$ats_active = 'ie6';
};
break;
case strpos($user_agent, 'MSIE 7') :
if ($data['ie7_switch'] == 1 && !empty($data['ie7_theme'])) {
$ats_active = 'ie7';
};
break;
case strpos($user_agent, 'MSIE 8') :
if ($data['ie8_switch'] == 1 && !empty($data['ie8_theme'])) {
$ats_active = 'ie8';
};
break;
default :
$ats_active = 0;
}
// Dev Mode Overide
if($data['dev_mode'] == 1) {
$ats_active = 1;
}
return $ats_active;
}
// Run active agents check
checkActive($ats_plugin);
if(!empty($ats_active)) {
function change_theme() {
global $ats_plugin;
global $ats_active;
global $the_theme;
if(!empty($ats_plugin['ie5_theme'])) {$ie5 = $ats_plugin['ie5_theme'];} else {$ie5 = null;}
if(!empty($ats_plugin['ie6_theme'])) {$ie6 = $ats_plugin['ie6_theme'];} else {$ie6 = null;}
if(!empty($ats_plugin['ie7_theme'])) {$ie7 = $ats_plugin['ie7_theme'];} else {$ie7 = null;}
if(!empty($ats_plugin['ie8_theme'])) {$ie8 = $ats_plugin['ie8_theme'];} else {$ie8 = null;}
$theme_key = array(
'ie5' => $ie5,
'ie6' => $ie6,
'ie7' => $ie7,
'ie8' => $ie8,
);
// Only one value should return
foreach ($theme_key as $browser => $selection ) {
if ($ats_active == $browser) {
$the_theme = $selection;
}
}
// Add the dev mode override
if(!empty($ats_plugin['dev_theme'])) {
$the_theme = $ats_plugin['dev_theme'];
}
return $the_theme;
}
add_filter('template', 'change_theme');
add_filter('option_template', 'change_theme');
add_filter('option_stylesheet', 'change_theme');
}
}
// For non-IE browsers, we check if the user is an admin and enable developer mode
if ( !$is_IE && current_user_can('manage_options') ) {
if($ats_plugin['dev_mode'] == 1 && !empty($ats_plugin['dev_theme'])) {
$the_theme = $ats_plugin['dev_theme'];
}
function dev_theme() {
global $the_theme;
return $the_theme;
}
/* #todo if the theme is in developer mode, there should be a visual warning as a reminder */
if ($ats_plugin['dev_mode'] == 1) {
add_filter('template', 'dev_theme');
add_filter('option_template', 'dev_theme');
add_filter('option_stylesheet', 'dev_theme');
}
}
I've been staring at this for too long... it's completely inefficient. I'm literally running the entire switch statement once, then checking if active is flagged, then I'm running it again. That's so very wrong. I desperately need a bit of perspective on the right way to structure the logic.
Background Information:
It's switching the theme based on a user agent as configured in a WP Plugin admin. The theme has a dev mode for configuring the plugin and an active mode for the user agents.
$ats_plugin grabs the Redux array
$the_theme grabs the selected theme from that array
$active grabs the switch for whether or not a theme is being served to that user agent.
The whole mess is being filtered back into WordPress on the end.
Here's my mess of code. It works but it's horribly inefficient.
Original Madhouse Logic:
<?php
/**
* Define some early logic, which should probably be moved to a class later
*/
// Initialize the main variables
$active = 0;
$the_theme = null;
function swapTheme()
{
// Fetch the plugin options array
global $ats_plugin;
global $the_theme;
global $active;
// Fetch the user agent
$user_agent = $_SERVER['HTTP_USER_AGENT'];
// Assign variables based on user agent
switch ($user_agent) {
case strpos($user_agent, 'MSIE 5') :
$active = $ats_plugin['ie5_switch'];
$the_theme = $ats_plugin['ie5_theme'];
break;
case strpos($user_agent, 'MSIE 6') :
$active = $ats_plugin['ie6_switch'];
$the_theme = $ats_plugin['ie6_theme'];
break;
case strpos($user_agent, 'MSIE 7') :
$active = $ats_plugin['ie7_switch'];
$the_theme = $ats_plugin['ie7_theme'];
break;
case strpos($user_agent, 'MSIE 8') :
$active = $ats_plugin['ie8_switch'];
$the_theme = $ats_plugin['ie8_theme'];
break;
default :
// Check for Developer Mode
if ($ats_plugin['dev_mode'] == 1) {
$active = 1;
// Check for Developer Theme
if (!empty($ats_plugin['dev_theme']) && current_user_can('manage_options')) {
$the_theme = $ats_plugin['dev_theme'];
} else {
$the_theme = null;
}
}
break;
}
return $the_theme;
}
swapTheme();
if (!empty($the_theme) && $active == 1) {
add_filter('template', 'swapTheme');
add_filter('option_template', 'swapTheme');
add_filter('option_stylesheet', 'swapTheme');
}
I'm putting my super humble student goggles on right now.
Screenshots of what I'm doing

Answer 1

OK, first of all, do not run swapTheme() outside of any filter. Second, just check if you already matched something and than simple return.
function swapTheme() {
if ( ! empty( $the_theme ) ) {
return;
}
// ...
}

Answer 2

While it's not exactly the answer I was hoping for, further iteration and a bit of banter with the guys at Redux Framework got things into a working state. The Alternate Theme Switcher code is now at Github if you'd like to see how I resolved the myriad issues that came up. I'd still like to further refactor things going forward but the question of "how to fix broken?" no longer needs to be active here.
Thanks for the input that was provided.