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

First, define your Google Analytics constant in wp-config.php:

// wp-config.php

/\* Google Analytics Key \*/
define('GOOGLE\_ANALYTICS\_KEY', 'G-XXXXXXXXXX');

Then, insert the following gist inside the <head> of your website (header.php in my case):

// header.php (for example)

<?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'))

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