send serverside conversions from a Wordpress shop (e. g. wooCommerce) to Google Analytics

functions.php

<?php
// add one of the two following options to the functions.php of your child theme in order to send server side 
// conversion data to Google Analytics
// NOTE: function store_gclid() is used by both variants

/***************************************************************************************************/
// send conversion to Analytics - Option 1: track success page as only pageview in the session and 
// create a goal for path "/conversion/" 
// adjust goal path (fragment) in $goalurl in order to fit your url structure
// comment out line below to deactivate
/***************************************************************************************************/

add_action ('wp_head','trackPages');

function trackPages() {

  //store gclid on landing page if present
  store_gclid();

  $gclid = $_SESSION['gclid'];
  $dp = $_SERVER['REQUEST_URI'];
  $goalurl = '/shop/checkout/order-received/';
  $isTrackPage = (strpos($dp, $goalurl) > -1);

  //track hit for conversion page if success page is loaded
  if (($isTrackPage === TRUE) && (isset($gclid) && $gclid !== "")) {
    $cid = $gclid;
    $dh = $_SERVER['SERVER_NAME'];
    $dp = urlencode("/conversion/?gclid=".$gclid);
    //Test:
    $dl = urlencode('https://'.$_SERVER['SERVER_NAME']."/conversion/?gclid=".$gclid);
    
    $pg_url = "https://www.google-analytics.com/collect?v=1&dh=$dh&dl=$dl&t=pageview&tid=UA-143498129-2&cid=$cid&aip=1";
    $ch = cURL_init();
    curl_setopt($ch, CURLOPT_URL, $pg_url);
    cURL_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    cURL_setopt($ch, CURLOPT_FRESH_CONNECT, true);
    cURL_setopt($ch, CURLOPT_TIMEOUT_MS, 200);
    curl_exec($ch);
    cURL_close($ch);
    return true;
  }
}

function store_gclid() {
  session_start(['cookie_secure' => true, 'cookie_httponly' => true, 'cookie_samesite' => true]);
  $gclid = $_GET['gclid'];
  if (isset($gclid) && $gclid !== "") {
  	$_SESSION['gclid'] = $gclid;
  }
}


/***************************************************************************************************/
// send conversion to Analytics - Option 2: use woocommerce hook and send event incl. sales amount
// uncomment both lines below to activate   
/***************************************************************************************************/

//add_action('woocommerce_thankyou', 'serverside_conversion_event');
//add_action ('wp_head','store_gclid');

function send_conversion_event($t_ua, $t_ea, $t_vl) {
  $gclid = $_SESSION['gclid'];
  if (isset($gclid) && $gclid !== "") {
    $cid = $gclid;
    $dh = $_SERVER['SERVER_NAME'];
    $dp = urlencode("/conversion/?gclid=".$gclid);
    $t_ea = urlencode($t_ea);
    $event_url = "https://www.google-analytics.com/collect?v=1&dp=$dp&dh=$dh&t=event&tid=$t_ua&cid=$cid&ec=Conversion&ea=$t_ea&ev=$t_vl&ni=0&aip=1";
    $ch = cURL_init();
    curl_setopt($ch, CURLOPT_URL, $event_url);
    cURL_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    cURL_setopt($ch, CURLOPT_FRESH_CONNECT, true);
    cURL_setopt($ch, CURLOPT_TIMEOUT_MS, 200);
    curl_exec($ch);
    cURL_close($ch);
    return true;
  } else return false;
}

function serverside_conversion_event($order_id) {
  $order = wc_get_order($order_id);
  send_conversion_event("UA-143498129-2", "Abschluss", $order->get_total());
}


?>

see https://www.markus-baersch.de/blog/serverside-google-ads-conversiontracking/ (german language) for more details