How to override wordpress core php files.

There is absolutely no recommended way of overwriting core files, and we haven’t had to do this in more than 7 years of working with WordPress. There are so many hooks it’s unbelievable, and anything that doesn’t have a hook could still be included in some way through custom PHP in your theme or a plugin.

This isn’t just moderators being tentative or holding back – there is literally no reason to hack the core on a production site. If WordPress really doesn’t achieve what you need it to, then it’s the wrong solution to the problem you’re trying to solve. (But we’ve also built some really advanced stuff in WordPress so that’s not always the case either.)

We build our themes and plugins using PHP classes, but I’ll exclude that so as not to complicate the issue…

Let’s say you want to integrate the Mailchimp API so that visitors can sign up to your email newsletter through your site, but you don’t want to send them to the Mailchimp signup form and you don’t want to use a plugin. There are many ways to accomplish this, but as an example:

You download the Mailchimp PHP SDK and put that into a folder in your theme (let’s call that folder mailchimp). Now, your theme folder looks something like this:

footer.php
functions.php
header.php
index.php
style.css
mailchimp/

When someone clicks your “Subscribe” button, you want to capture that information, connect with Mailchimp via the SDK, and then continue loading your site after adding the user to your list. To do this, you need to include (for example) the init.php file in the mailchimp directory when the form is submitted:

add_action( 'init', 'my_mailchimp_subscribe' );
function my_mailchimp_subscribe(){
  // If the form was submitted with the EMAIL variable
  // let's load the mailchimp file and add the user
  if( $_POST && is_email( $_POST[ 'EMAIL' ] ) ){
    // You're in a custom theme. If mailchimp is in a child theme
    // you'd use get_stylesheet_directory() instead
    require_once( get_template_directory() . '/mailchimp/init.php' );

    // Mailchimp is now loaded and we can add the user
    ...
  }
}

And that’s it. You can put whatever you want in your theme (or a plugin) and then include it when you need it. You could use WordPress hooks to determine when to include code and when not to so you don’t slow your site down, loading resources when they’re not needed.

I put the require_once inside the function so it’s only called when the function is run, because I need it on demand at that time but I don’t need it to load, for example, when someone is just viewing the home page. Why slow the site down?

I prefer require_once because it first checks if the file has been required and, if so, doesn’t bring it in again. I prefer require/require_once over include/include_once because require/require_once will kill my script if it runs into an error vs. include/include_once which just generates a warning.

During development, I want to write bug-free code. I’d rather my site/application die/end/freeze during development so I can debug it and get it right before I release it into production. Plus, I find it makes finding the error easier because if I require something, hit save, reload, and my site crashes, I know what the problem is and I’m not banging my head against a wall later wondering why it’s not working when some function I’m trying to access doesn’t exist because it was never actually loaded in!