Adds basic feature of Google Tag Manager to MarketPress

[MarketPress] - Google Tag Manager

<?php
/*
Plugin Name: MarketPress - Google Tag Manager
Plugin URI: https://premium.wpmudev.org/
Description: Adds Google Tag Manager to site
Author: Panos Lyrakis @ WPMUDEV
Author URI: https://premium.wpmudev.org/
License: GPLv2 or later
*/

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

if ( ! class_exists( 'WPMUDEV_MP_Google_Tag_Manager' ) ) {

	class WPMUDEV_MP_Google_Tag_Manager {

        private static $_instance = null;
        //private static $_gtm_key = 'GTM-XXXXXXX';
        private static $_ga_key = 'UA-XXXXXXXXX-X';
        
		public static function get_instance() {

			if( is_null( self::$_instance ) ){
				self::$_instance = new WPMUDEV_MP_Google_Tag_Manager();
			}
            return self::$_instance;
            
		}
		private function __construct() {

            add_filter( 'mp_order/header', array( $this, 'load_tag_manager_head_script' ), 0 );

        }

        public function load_tag_manager_head_script( $html ){

            $order = new MP_Order( get_query_var( 'mp_order_id' ) );
            //if order not exist, just return false
            if ( ! $order->exists() || $order->get_meta( 'mp_ga_tracked' ) ) {
                return $html;
            }

            $order = apply_filters( 'mp_ga_ecommerce', $order );

            return $this->_ga_send_order( $order ) . $html;

        }

        public function _ga_send_order( $order = null ){

            if( is_null( $order ) ){
                return;
            }

            $cart = $order->get_meta( 'mp_cart_info' );
            
            $products = $cart->get_items_as_objects();

            if( empty( $products ) ){
                return;
            }

            ob_start();
            ?>

            <!-- Google Analytics -->
            <script>
            
            (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
            (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
            m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
            })(window,document,'script','https://www.google-analytics.com/analytics.js','ga');

            ga('create', '<?php echo self::$_ga_key; ?>', 'auto');
            ga('send', 'pageview');
            ga('require', 'ecommerce');   // Load the ecommerce plug-in.

            ga('ecommerce:addTransaction', {
            'id':               '<?php echo $order->ID ?>',                                     // Transaction ID. Required
            'affiliation':      '<?php echo get_bloginfo('name'); ?>',                          // Affiliation or store name
            'revenue':          '<?php echo $order->get_meta( 'mp_order_total' ); ?>',          // Grand Total
            'shipping':         '<?php echo $order->get_meta( 'mp_shipping_total' ); ?>',       // Shipping
            'tax':              '<?php echo $order->get_meta( 'mp_tax_total' ); ?>'             // Tax
            });
            
            <?php foreach( $products as $product ){ ?>

                <?php
                $product = new MP_Product( $product->ID );
                $sku = esc_attr( $product->get_meta( 'sku' ) );
                $categories = array();
                $terms = get_the_terms( $product->ID, 'product_category' );
                if( ! empty( $terms ) ){
                    foreach( $terms as $term ){
                        $categories[] = $term->name;
                    }                    
                }
                
                ?>

                ga('ecommerce:addItem', {
                'id':           '<?php echo $product->ID; ?>',                              // Transaction ID. Required
                'name':         '<?php echo $product->title( false ); ?>',                  // Product name. Required
                'sku':          '<?php echo !empty( $sku ) ? $sku : $product->ID; ?>',      // SKU/code
                'category':     '<?php echo implode( ',', $categories ); ?>',               // Category or variation
                'price':        '<?php echo $product->get_price( 'lowest' ); ?>',           // Unit price
                'quantity':     '<?php echo $cart->get_item_qty( $product->ID ); ?>'        // Quantity
                });

            <?php } ?>

            ga('ecommerce:send');      // Send transaction and item data to Google Analytics.

            </script>

            <?php
            
            $ga_ecommerce = ob_get_clean();
            add_post_meta( $order->ID, 'mp_ga_tracked', true, true );

            return $ga_ecommerce;
        }

    }

    add_action( 'plugins_loaded', function(){
		$GLOBALS['WPMUDEV_MP_Google_Tag_Manager'] = WPMUDEV_MP_Google_Tag_Manager::get_instance();
    }, 10 );
    
}