Converting core modification to a plugin

Question

I needed to change two strings shown to the user on the site upon clicking the confirmation links in the email (not to modify the emails sent out). I made the following modifications and it worked.:

    --- wp-activate.bak        2017-02-28 13:01:03.883175677 +0530
    +++ wp-activate.php     2017-02-28 13:47:35.000000000 +0530
    @@ -13,6 +13,10 @@ require( dirname(__FILE__) . '/wp-load.p

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

    +// Changes the header
    +
    +require( dirname(__FILE__) . '/wp-custommsg.php' );
    +
     if ( !is_multisite() ) {
            wp_redirect( wp_registration_url() );
            die();
    @@ -128,7 +132,7 @@ get_header( 'wp-activate' );
                            $url = isset( $result['blog_id'] ) ? get_home_url( (int) $result['blog_id'] ) : '';
                            $user = get_userdata( (int) $result['user_id'] );
                            ?>
    -                       <h2><?php _e('Your account is now active!'); ?></h2>
    +                       <h2><?php _e($active_page_title); ?></h2>

                            <div id="signup-welcome">
                                    <p><span class="h3"><?php _e('Username:'); ?></span> <?php echo $user->user_login ?></p>
    @@ -147,7 +151,7 @@ get_header( 'wp-activate' );
                            <?php else: ?>
                                    <p class="view"><?php
                                            /* translators: 1: login URL, 2: network home URL */
    -                                       printf( __( 'Your account is now activated. <a href="%1$s">Log in</a> or go back to the <a href="%2$s">homepage</a>.' ), network_site_url( 'wp-login.php', 'login' ), network_home_url() );
    +                                       printf( __( $active_page_msg ), network_site_url( 'wp-login.php', 'login' ), network_home_url() );
                                    ?></p>
                            <?php endif;
                    }

And wp-custommsg.php contains:

<?php

$active_page_title = 'My custom title';
$active_page_msg = 'My custom message';
?>

I tried to read about hooks and actions but can't seem to wrap my head around them. How can I convert my changes into a plugin so that it survives a wordpress version update?

To clarify, these modifications reside in html code outside of the php content in wp-activate.php, and as far as I can see, do not have hooks.

Answer 1

I haven't done this kind of a hook yet but I did find this page in the wordpress codex

https://codex.wordpress.org/Function_Reference/wpmu_welcome_user_notification

They do explain a bit their about how you can modify the content of the activation and notifcation emails

I will try to explain a bit here In Wordpress an Action is a type of hook. You get Action Hooks and Filter Hooks

Actions are triggered by specific events that happen in Wordpress like when a post is published you can trigger a custom action. Linked to a function that handles what you need it to do. An idea here is say for instance you would like to update a third party database when a specific post is updated you could link that to an action. Actions are hooks to Wordpress using add_action()

Filters are functions defined for Wordpress to run data through as it is displayed. So say for instance you would like to change some text on a post without affecting the database you would use a filter. That way when you change the filter you don't have to edit the actual database values of the posts. Filters are hooked into wordpress using add_filter()

For more information I think have a read at this page https://codex.wordpress.org/Plugin_API which explains more in details about Hooks.