Improve WP Signup - redirect to the main network URL with referral parameters

improve-wp-signup-redirect.php

<?php
/**
 * @package Improve WP Signup - redirect to the main network URL with referral parameters
 * @author  James Morrison
 *
 * The WP core /wp-signup.php file calls "get_header( 'wp-signup' );" we can hook into this to force a redirect
 * Add this as a MU Plugin, make sure NOBLOGREDIRECT is *not* defined as it'll override this
 *
 * Notes:
 * Whilst you can set up a redirect from the NOBLOGREDIRECT constant, the advantage of using this
 * is that you can capture the path that triggered the call to signup; as seen in the example below,
 * this uses Google Analytics parameters to help track where the requests came from without
 * the need to setup infrastructure level redirects.
 *
 * Additionally, NOBLOGREDIRECT has a nasty habit of messing about with 404 pages - in some cases
 * it treats these as an invalid network site and triggers the redirect instead of showing the 404 page
 *
 **/

// Security check
defined( 'ABSPATH' ) || die();

// Hook into the header action to redirect
add_action( 'get_header', function( $name ) {

    if ( 'wp-signup' == $name ) {

        wp_redirect(
            esc_url_raw(
                add_query_arg( [
                    'utm_source'   => isset( $_GET[ 'new' ] ) ? esc_attr( $_GET[ 'new' ] ) : '',
                    'utm_medium'   => 'referral',
                    'utm_campaign' => 'no-blog-redirect' 
                ], network_home_url() )
            ),
        301 );
        
        exit();

    }

}, 1, 1 );