Insert a Google Analytics sitetag in Wordpress using a constant defined in wp-config.php

wordpress-ga-sitetag.php

<?php
if (defined('GOOGLE_ANALYTICS_KEY')) {
?>
	<!-- Global site tag (gtag.js) - Google Analytics -->
	<script async src="https://www.googletagmanager.com/gtag/js?id=<?php echo GOOGLE_ANALYTICS_KEY; ?>"></script>
	<script>
		window.dataLayer = window.dataLayer || [];

		function gtag() {
			dataLayer.push(arguments);
		}
		gtag('js', new Date());

		gtag('config', '<?php echo GOOGLE_ANALYTICS_KEY; ?>');
	</script>
<?php
} // if (defined('GOOGLE_ANALYTICS_KEY'))