Sample WordPress theme functions.php modification to generate Client ID

functions.php

// Source: https://www.simoahava.com/analytics/google-analytics-client-id-amp-pages/
// REST API for GTM container
add_action( 'rest_api_init', function() {
    register_rest_route( 
        'amp-gtm',
        '/amp.json',
        array(
            'methods' => 'GET',
            'callback' => 'retrieve_gtm_json',
        )
    );
});

// Generate random Client ID
function generate_ga_client_id() {
    return rand(100000000,999999999) . '.' . time();
}

// Set cookie to expire in 2 years
function getCookieExpirationDate() {
    return date('D, j F Y H:i:s', time() + 60*60*24*365*2);
}

// Callback for the GET request
function retrieve_gtm_json( $data ) {
    /* Get the hostname of the request origin, and parse it for the
     * pure domain name. */
    $domain = explode(':', $data->get_header('Host'))[0];
    $domainName = str_replace('www.', '', $domain);

    // Get the number of parts in the domain name
    $domainLength = count(explode('.', $domainName));

    /* Check if the browser already has the _ga cookie.
     * If not, generate a new cookie. */
    $cid = $_COOKIE['_ga'];
    if (!isset($cid)) {
        $cid = "GA1.{$domainLength}." . generate_ga_client_id();
    }

    /* Store the actual Client ID (last two numbers) of the
     * _ga cookie value in the $cidNumber variable */
    $cidNumber = preg_replace('/^GA.\.[^.]+\./','',$cid);
  
    // Get all HTTP request parameters
    $query = $_SERVER['QUERY_STRING'];
    
    /* Fetch the actual GTM container, by passing the valid query parameters from
     * the original request. */
    $container = file_get_contents("https://www.googletagmanager.com/amp.json?{$query}");
    
    // Replace the CLIENT_ID(AMP_ECID_GOOGLE) string with ${clientId}
    $container = str_replace('CLIENT_ID(AMP_ECID_GOOGLE)','${clientId}', $container);
  
    // Add the clientId to the "vars" object in the container JSON.
    $container = json_decode($container);
    $container->vars->clientId = $cidNumber;
    
    // Build a new HTTP response from the modified configuration file.
    $response = new WP_REST_RESPONSE( $container );
    
    // Add the required headers (Set-Cookie, most importantly) to the Request
    $response->header( 'Set-Cookie', "_ga={$cid}; Path=/; Expires=" . getcookieExpirationDate() . " GMT; Domain={$domainName};");
    $response->header( 'Access-Control-Allow-Origin', 'https://cdn.ampproject.org');
    
    // Remember to check the protocol and change to http if that's where your domain is
    $response->header( 'AMP-Access-Control-Allow-Source-Origin', "https://{$domain}");
    $response->header( 'Access-Control-Expose-Headers', 'AMP-Access-Control-Allow-Source-Origin');
    
    // Return the HTTP response.
    return $response;
}