Basic GTM WP Plugin

<?php
/*
Plugin Name: GTM
Description: Inserts GTM tracking code
Version: 1.0
Author: Jose Cajide
Author URI: http://es.linkedin.com/in/jrcajide/
Plugin URI: http://es.linkedin.com/in/jrcajide/
*/


add_filter('template_include','gtm_include',1);
function gtm_include($template) {
    ob_start();
    return $template;
}
add_filter('shutdown','gtm_code',0);
function gtm_code() {
    $insert =  "<!-- Google Tag Manager -->
<noscript><iframe src='//www.googletagmanager.com/ns.html?id=GTM-XXXX'
height='0' width='0' style='display:none;visibility:hidden'></iframe></noscript>
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'//www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXX');</script>
<!-- End Google Tag Manager -->";
    $content = ob_get_clean();
    $content = preg_replace('#<body([^>]*)>#i',"<body$1>{$insert}",$content);
    echo $content;
}


?>