WordPress User ID tracking for GA4 using gtag.js API

add-config-gtag-user-id-functions.php

<?php /* Leave this line out when adding the below code to child theme functions.php */

/** This PHP hook supports GA4 tracking of the WordPress user ID. */
add_action( 'wp_head', function () {
  // REMEMBER TO ADD YOUR MEASUREMENT ID BELOW
  $gtag_config = "gtag('config', 'G-ABCDEFGHIJ');";
  // If someone's logged in, let's track.
  if ( is_user_logged_in() ) {
	$gtag_config = "gtag('config', 'G-ABCDEFGHIJ', { 'user_id': '" . get_current_user_id() . "' });";
	// Set the user_id custom dimension to allow recording and reporting in GA4. NOT NEEDED ANYMORE
	//$gtag_config .= "gtag('set', 'user_properties', { 'user_id': '" . get_current_user_id() . "' });";
  }
 
?>

  <!-- Global site tag (gtag.js) - Google Analytics - REMEMBER TO CHANGE THE MEASUREMENT ID -->
  <script async src="https://www.googletagmanager.com/gtag/js?id=G-ABCDEFGHIJ"></script>
  <script>
    window.dataLayer = window.dataLayer || [];
    function gtag(){dataLayer.push(arguments);}
    gtag('js', new Date());
    <?php echo $gtag_config ?>
  </script>

<?php 

  // Let's send an event to test the custom dimension set above. Assumes you have a "contact-us" page.
  if( is_page( array( 'contact-us' ) ) ) {
    // Most contact pages have a form. So, let's send a form impression event. 
    // If someone's logged in, the "user property" (custom dimension) is sent 
    // with this event.
    echo "<script>gtag( 'event', 'FormImpression', { 'FormName': 'Contact Us' } );</script>";
  }
									
} );