gtm4wp // custom add to cart button

<?php

/**
 * Custom add to cart button for WooCommerce.
 * This version is compatible with gtm4wp plugin.
 */

add_shortcode( 'gtm4wp_add_to_cart', 'gtm4wp_add_to_cart_shortcode' );

function gtm4wp_add_to_cart_shortcode( $atts ) {
  $a = shortcode_atts( array(
    'id'        => '',
    'quantity'  => 1,
    'text'      => __( 'Add to cart', 'woocommerce' )
  ), $atts );

  if( ! $a['id'] ) {
    return "{ error: missing shortcode arguments }";
  }

  global $product;
  $product = new WC_Product( intval( $a['id'] ) );

  ob_start();
  ?>

  <form class="cart" action="<?php echo esc_url( wc_get_cart_url() . "?add-to-cart={$product->id}&quantity={$a['quantity']}" ); ?>" method="post" enctype="multipart/form-data">
    <button type="submit" name="add-to-cart" value="<?php echo intval( $a['id'] ); ?>" class="single_add_to_cart_button button alt">
      <?php esc_html_e( $a['text'] ); ?>
    </button>

    <input type="hidden" name="quantity" value="<?php echo intval( $a['quantity'] ); ?>">
    <?php echo do_action( 'woocommerce_after_add_to_cart_button' ); ?>
  </form>

  <?php
  return ob_get_clean();
}