wpmu_activate_signup( string $key )

function wpmu_activate_signup($key) {
	global $wpdb;

	$signup = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $wpdb->signups WHERE activation_key = %s", $key) );

	if ( empty( $signup ) )
		return new WP_Error( 'invalid_key', __( 'Invalid activation key.' ) );

	if ( $signup->active ) {
		if ( empty( $signup->domain ) )
			return new WP_Error( 'already_active', __( 'The user is already active.' ), $signup );
		else
			return new WP_Error( 'already_active', __( 'The site is already active.' ), $signup );
	}

	$meta = maybe_unserialize($signup->meta);
	$password = wp_generate_password( 12, false );

	$user_id = username_exists($signup->user_login);

	if ( ! $user_id )
		$user_id = wpmu_create_user($signup->user_login, $password, $signup->user_email);
	else
		$user_already_exists = true;

	if ( ! $user_id )
		return new WP_Error('create_user', __('Could not create user'), $signup);

	$now = current_time('mysql', true);

	if ( empty($signup->domain) ) {
		$wpdb->update( $wpdb->signups, array('active' => 1, 'activated' => $now), array('activation_key' => $key) );

		if ( isset( $user_already_exists ) )
			return new WP_Error( 'user_already_exists', __( 'That username is already activated.' ), $signup);

		/**
		 * Fires immediately after a new user is activated.
		 *
		 * @since WP-MU (3.0.0)
		 *
		 * @param int   $user_id  User ID.
		 * @param int   $password User password.
		 * @param array $meta     Signup meta data.
		 */
		do_action( 'wpmu_activate_user', $user_id, $password, $meta );
		return array( 'user_id' => $user_id, 'password' => $password, 'meta' => $meta );
	}

	$blog_id = wpmu_create_blog( $signup->domain, $signup->path, $signup->title, $user_id, $meta, get_current_network_id() );

	// TODO: What to do if we create a user but cannot create a blog?
	if ( is_wp_error($blog_id) ) {
		// If blog is taken, that means a previous attempt to activate this blog failed in between creating the blog and
		// setting the activation flag. Let's just set the active flag and instruct the user to reset their password.
		if ( 'blog_taken' == $blog_id->get_error_code() ) {
			$blog_id->add_data( $signup );
			$wpdb->update( $wpdb->signups, array( 'active' => 1, 'activated' => $now ), array( 'activation_key' => $key ) );
		}
		return $blog_id;
	}

	$wpdb->update( $wpdb->signups, array('active' => 1, 'activated' => $now), array('activation_key' => $key) );
	/**
	 * Fires immediately after a site is activated.
	 *
	 * @since WP-MU (3.0.0)
	 *
	 * @param int    $blog_id       Blog ID.
	 * @param int    $user_id       User ID.
	 * @param int    $password      User password.
	 * @param string $signup_title  Site title.
	 * @param array  $meta          Signup meta data. By default, contains the requested privacy setting and lang_id.
	 */
	do_action( 'wpmu_activate_blog', $blog_id, $user_id, $password, $signup->title, $meta );

	return array('blog_id' => $blog_id, 'user_id' => $user_id, 'password' => $password, 'title' => $signup->title, 'meta' => $meta);
}