1<?php
  2
  3add_hook('ClientAreaPage', 1, function($vars) {
  4
  5    if (!empty($_GET['gclid'])) {
  6        $gclid = trim((string)$_GET['gclid']);
  7
  8        $_SESSION['gclid'] = $gclid;
  9
 10        // keep long enough to survive “trial then later login”
 11        setcookie('gclid', $gclid, time() + (7 * 86400), "/");
 12    }
 13});