Stuck with WordPress Multisite Custom signup page without .htaccess modification

Yes, stuck with creating a custom signup page for WordPress Multisite.

It's a well revised question, and many of us is seeking the answer. May be somebody got one, but still, I can't find any solid, holistic one for the issue. I'm trying to find a non .htaccess way - a PHP' or a WordPress' way.

Step 1:

I studied the most obvious article found in this quest:

• Using a Custom wp-signup Page With WordPress MU - Chris Wiegman

Okay, now I've copied all the code of /wp-signup.php to a file in my theme named signup.php, make it a page template and called the page with the URL http://example.com/signup.

I made changes to the required places. Like instead of get_header( 'wp-signup' ); I used my custom one from my theme get_header('null');. That one (header-null.php) is pretty bare-bone in design and architecture from the default header.php file of my theme.

Alas! There is no header-wp-signup.php in anywhere of WordPress

Similarly I replaced get_footer( 'wp-signup' ); with:

    ? !-- /ending of php tag --
    /div !-- /.container .site-area .not-logged-in-page --

?php wp_footer(); ?

/body
/html

Step 2:

@toscho helped me solving two issues of recalling files that are already loaded. So, instead of showing absolute paths, I simply commented out the lines:

//require( dirname(__FILE__) . '/wp-load.php' );

and

//require( dirname( __FILE__ ) . '/wp-blog-header.php' );

Now I've a nice, working file for newly modified sign-up page. I need to drive the traffic from /wp-signup.php to here, not using .htaccess.

Step 3:

I found this WordPress.org Support thread very helpful:

• individual MU Registration Page possible? - David Sader

Now I made a mu-plugin making a file in: wp-content/mu-plugins/sign-up-location.php. There I used two functions:

/**
 * Changing the Registration page URL.
 * 
 * @return string The URL of the new page.
 * --------------------------------------------------------------------------
 */
function project_signup_page() {
    $page =  network_site_url() .'signup/';
    return $page;
}
add_filter( 'wp_signup_location', 'project_signup_page' );


/**
 * Redirect from default signup page to custom page.
 * 
 * @return void
 * --------------------------------------------------------------------------
 */
function project_redirect_from_default_signup() {
    if( strpos($_SERVER['REQUEST_URI'], 'wp-signup.php')) {
        wp_redirect(network_site_url() .'signup/');
        exit();
    }
}
add_action( 'plugins_loaded', 'project_redirect_from_default_signup' );

The first one is changing the registration URL to my custom path. And the second one is my desired one, driving traffic from /wp-signup.php to /signup without .htaccess.

Note: Instead of creating MU Plugin, you can get the same affect using a regular plugin now.

Step 4 (furnishing steps):

As per @toscho's suggestion I replaced the page template thing with a Rewrite Endpoint:

?php
/**
 * Rewrite endpoint : /register.
 *
 * @return void
 * --------------------------------------------------------------------------
 */
function project_registration_endpoint() {
    add_rewrite_endpoint( 'register', EP_ROOT );
    if( get_option(EDIT_REWRITE_RULE) != 1 ) {
        flush_rewrite_rules();
        update_option( EDIT_REWRITE_RULE, 1 );
    }
}
add_action( 'init', 'project_registration_endpoint' );

/**
 * Custom Template for Registration.
 *
 * @param  string $template Template to be loaded.
 * @return string           Our custom template to show up.
 * --------------------------------------------------------------------------
 */
function project_custom_templates( $template ) {
    if( ! is_admin() ) {
        if( get_query_var('register') || strpos($_SERVER['REQUEST_URI'], 'wp-signup.php') ) {
            $template = get_template_directory() .'/signup.php';
        }
    }
    return $template;
}
add_filter( 'template_include', 'project_custom_templates' );

/**
 * Return a value for '/register'
 * Make sure that 'get_query_var( 'register' )' will not return just an empty string if it is set.
 *
 * @link   http://wordpress.stackexchange.com/a/51450/22728
 * @link   https://www.pmg.com/blog/a-mostly-complete-guide-to-the-wordpress-rewrite-api/
 *
 * @param  array $vars  Query variables.
 * @return array        Modified query variables.
 * --------------------------------------------------------------------------
 */
function project_set_epex_var( $vars ) {
    isset( $vars['register'] ) and $vars['register'] = true;
    return $vars;
}
add_filter( 'request', 'project_set_epex_var' );

Modified: And at that point, I changed the slug /signup to /register to resolve any possible conflict, that can happen. I changed the mu-plugin's strings accordingly. And make the page template retired.

Everything is working fine, and I got my desired thing :) ... except...

the success or error messages...
all the success or error messages are gone with wp_redirect(). :o

I checked my file, yes, the show_user_form() is there with $errors parameter, everything is fine, except the error or success messages.

How can I display the success and/or error messages in my custom page signup page?

In Bengali

I explained the whole thing in details with all the sources in Bengali, in my blog post - I'm afraid, automatic translation to Bengali is till not that good.