Googleタグマネージャーを functions.phpで読み込ませる（header.phpには書き込まない）

functions.php

// Google Tag Manager
<?php
function add_gtm_head() {
  echo "<!-- 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-XXXXXXXX');</script>
  <!-- End Google Tag Manager -->";
}
add_action('wp_head', 'add_gtm_head', 0);

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

add_action('wp_body_open', 'add_gtm_body', 0);

?>