add-gtm.php

<?php
/*
Plugin Name: Añadir el script de Google Tag Manager
Plugin URI: http://carlosmdh.es
Description: Plugin para insertar los Scripts de Google Tag Manager
Version: 1.0.2
Author: carlosmdh
License: GPL 2+
License URI:
*/

//Añadir el código de Google Tag Manager en el <head>//

add_action( 'wp_head', 'carlosmdh_google_tag_manager1' );

function carlosmdh_google_tag_manager1() { ?>

    <!-- Google Tag Manager -->
    <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=
        'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
    })(window,document,'script','dataLayer','GTM-WDV8SRG');</script>
    <!-- End Google Tag Manager -->

<?php }

/// Añade el código de Google Tag Manager code justo antes de la de la etiqueta de cierre </body> //

add_action( 'wp_footer', 'carlosmdh_google_tag_manager2' );

function carlosmdh_google_tag_manager2() { ?>

    <!-- Google Tag Manager (noscript) -->
    <noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-WDV8SRG"
    height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
    <!-- End Google Tag Manager (noscript) -->
<?php }