In a template, I have a need to include_once an external PHP (an include.php) that contains functions that I need to use inside the template.

Included in those functions are CSS styles and an external JS script.

The CSS is inside a function in the include.php. And the JS script also needs to be in the area.

So I need to include_once("includes.php") (which is in the theme's root folder). I figured I would use code similar to this:

add_action('wp_head', 'fst_include',20);
function fst_include() {
    include_once get_stylesheet_directory(). "/includes.php"; // load required functions

Since the action is 'wp_head', that should get the include_once("includes.php") in the head area. I use get_stylesheet_directory because the includes.php file will be in a Child Theme.

Once that is loaded, then I can add some CSS with this:

add_action('wp_head', 'more_css',30) // 'more_css' is a function inside the includes.php file that contains some CSS

And add some JS with

add_action('wp_head', 'some_script',30) // 'some_script' is a function inside the includes.php file that adds some JS

I give the last two add_actions a priority of '30' so they will be loaded after the includes.php file is loaded. (Because the includes.php has a priority of 20.)

But the includes.php file doesn't appear to be loading, as the 'some_script' function (contained in includes.php) is not available, and causes an error.

So I need a way to add a include_once('includes.php') so that the functions inside there are available 'later' in the template. I don't think that get_template_part is appropriate because the include_once needs to be in the section.

Added - But Why?

Why do I need to do this? My template displays a form. The form's code and functions are in the include.php file. Even the form is displayed by a function in the include.php.

The include.php file is a 'package' of functions and scripts and actions and changing of DOM elements to perform specific functions when the form is displayed - and the user fills out the form and submits it. (The 'package' is the process that is used to protect forms - like contact forms - against spam-bots. It's an effective process, but a bit complex. If you are really interested, you can go here to learn about it.)

The include.php file is also built as a standalone/non-WP code that can be used on non-WP pages. I don't want to maintain two versions of the code.

The form has to 'live' in the visual look of whatever theme is being used, if used on a WP site. I figured a template would be the best way to do that. So I need to get my include.php 'inside' the template so that it's functions can be called.

As for the priorty used in the add_action, that was an attempt to make sure that the include.php file is included in the area so that I can call the function that includes the CSS used for the form. If include.php is not loaded first, then the CSS-calling function will fail.

  • Wow. What exactly are you trying to achieve and why? Because including php files using actions with priorities looks like total over complication... Jan 13, 2019 at 22:57
  • @KrzysiekDróżdż we don't know what Rick was thinking, but I'm sure he had his reasons, so ask nicely and politely
    – Tom J Nowell
    Jan 13, 2019 at 23:16
  • @TomJNowell Tom, sorry if that was impolite. I’m just really surprised with that approach and it’s really, really strange - that’s why the question what is the point of that, so we can get the idea and be able to solve the problem and not only patch it... (TBH I don’t see anything impolite in this approach or in my comment about over complicating... but I’m not native speaker - so sorry again) Jan 13, 2019 at 23:44
  • Added "Why?" information. Jan 14, 2019 at 0:33

2 Answers 2


Including PHP files inside actions isn't recommended, and is considered highly unusual. If what you were trying to do worked, it would be fragile, and heavily dependent on ordering.

The reason your code didn't work:

add_action('wp_head', 'fst_include',20);

Is because actions are added with a priority of 10 by default, yet your include is on priority 20, so it happens after the other hooks, not before. As a result, when more_css is executed, it has yet to be defined.

Instead, don't load the file on a hook, just load it immediatley so that the functions are defined, but not executed, then add the functions as actions.

In general, it is a bad idea to mix loading code, and running code. Try to:

  • Load the code or autoloader
  • Create all the objects but don't run them
  • Then run it once everything is assembled

After much experimentation, and limited help from the googles, I figured how to do an 'include' of an external functions file inside a Template.

The key is to use the get_template_part in your Template at the beginning (after the template header). The file specified in that function must be in your Theme's root folder, although I think it could also be in the theme's template folder.


This will do the equivalent of a 'include' in a non-WP PHP code. It will 'include' the file called my_custom_functions.php .

Once you do that, then you can use filters to include things in the generated page's <head> area, or any other filters you need, as in:


Which will call the some_function() function in your my_custom_functions.php file. (Make sure that your function names are unique, of course.)

This process, which is not easily found via the googles, will let you 'include' a file and then use functions in your include file in your WP Template.

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