WordPress: add_filter or add_action? It actually doesn’t matter!

It’s a doctrine of WordPress plugin development:

Use actions to expand the functionality and filters to change information.

We’ve all heard it, we always follow that demand. Always? Well…

… if I want to add some functionality and nowhere near or far there is an action hook in the core but only a filter hook, I use that hook without considering the doctrine.

Surprisingly it works. But why? If we all insist that you have to use the right hook type, there has to be a difference between them. Let’s take a look behind the scenes (/wp-includes/plugin.php):

function add_action($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
   return add_filter($tag, $function_to_add, $priority, $accepted_args);
}

What? add_action does nothing but call add_filter and nothing else? There is no difference in functionality, it’s only a different name. So there is no reason why you shouldn’t use add_filter instead of add_action if you have no other solution.

I don’t say that you should interchange filter and action hooks at will. Using the correct hook type makes your code more readable and maintainable, for yourself and for others. Nevertheless, if there’s not other way to accomplish your goal but to use a filter hook instead of an action, don’t hesitate, it virtually makes no difference.