I am wondering if the below logic is correct and if certain circumstances should be present for it to work, since in some cases it does not seem to be working.

The example below is quite simple. Let's say that I want to use a function, that is defined elsewhere in a theme-related file, let's say parent_theme_hooks.php, through an action hook in my child-theme functions.php.


function is_enabled(){
    return true;

function check_if_enabled(){    
    do_action( 'my_hook', $some, $args );

Then in the child-theme functions.php

function my_function($some, $args) {
    if ( is_enabled() ) {
        $message = 'yes';
    } else {
        $message = 'no';
    echo $message;
add_action( 'my_hook', 'my_function', 11, 2 );

Question So my question is if I can use the function is_enabled() in the child-theme functions.php when it is defined elsewhere in the parent theme?


  • 1
    yes. if you have reason to believe the file with the function may not have loaded, you can use say if (function_exists('is_enabled') && is_enabled()), but if the do_action and the function are in the same file like this you will not have that problem. – majick Nov 28 '18 at 12:23
  • 1
    As long as a function is loaded when your hook is fired, you can use it. – butlerblog Nov 28 '18 at 12:24
  • Thanks for the function_exists that gives me an extra check. Actually the do_action and the function is_enabled are not in the same file. Just to be sure, when the functions are loaded, I should be able to call these functions in the child-theme functions.php correct? – RobbTe Nov 28 '18 at 12:26
  • You shouldn't need to use function_exists. That will just stop the code working if the function hasn't loaded, and what's the point of that? You shouldn't be trying to use it before it has loaded. The important thing is to understand when it has loaded, and only use it when it is safe to. – Jacob Peattie Nov 28 '18 at 12:27
  • @JacobPeattie Thanks. I understand that I should only use a function once it has loaded. That is why I am asking this. In the logic provided above, the is_enabled() function has been loaded correct? – RobbTe Nov 28 '18 at 12:32

Yes, you can. But you have to be careful, since some special cases may occur. Some of them are:

1. Function may be class method and not a global function:

class SomeClass {
   function some_function() { ... }

In such case, you can use it only if: - it's a static function - it's public method and you have access to some object of that class.

2. File with that function may not be included always:

function do_something() {
    if ( condition_met() ) {
        include_once( 'parent_theme_hooks.php' );

In such case the functions from that file won't be accessible always.

3. Function may be declared inside if statement:

if ( class_exists( 'SomeClass' ) ) {
    function some_function() { ... }

Again - if given class doesn't exist, then the function won't be accessible.

4. File is not loaded yet

If the function comes with a plugin or a theme, then it may be declared using some action hook.

In such case the function won't be accessible before that hook is fired up.

Another example of this case is using functions from parent theme inside child theme. functions.php file from parent theme comes after the same file from child theme. So you can't use parent functions directly in child themes functions.php file.


There are some ways you can make your code more reliable when using such functions:

  1. Always try to understand when and how is that function declared.
  2. Check if the function exists with if ( function_exists( 'function_name' ) ) and call it only if it does. You can also provide some alternative solution (fallback), if it doesn't exist.

You can use functions defined in the parent theme, but you can only use them after the parent theme is loaded. The parent theme is loaded after your child theme.

This means that if you want to use a function from the parent theme you can only do it inside a function that is hooked into a hook on after_setup_theme or later.

So your example will only work if check_if_enabled() is run after the parent theme has loaded. If you attempt to use that function in your child theme before the parent theme has loaded, it won't work.

  • Thanks, in the example above, the check_if_enabled() is a function in the parent theme so that should work. I was especially wondering if i can use the function is_enabled() through the my_hook action in the child-theme functions.php. Thanks – RobbTe Nov 28 '18 at 12:30
  • Only if my_hook is executed after the parent theme is loaded, and since that hook runs inside check_if_enabled() it depends entirely on when check_if_enabled() is run. – Jacob Peattie Nov 28 '18 at 12:35
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    As I have said, it depends on when you use check_if_enabled(). If you're not using it in your child theme, it's fine, if you are, then it needs to be used in a hook that runs after the parent theme has loaded. – Jacob Peattie Nov 28 '18 at 12:41
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    It does matter if the file that contains the definition has been included or not. If I define a function in a parent theme's functions.php file, and try to use it in the child theme outside a hook, I will get a "Call to undefined" error. This is because the parent theme's functions.php file has not been included yet. This applies to plugins too. If you try to use a function from a plugin that loads after yours you will get an error unless you're running the function after the plugins_loaded hook has run. – Jacob Peattie Nov 28 '18 at 13:38
  • 1
    That being said, you should not be running any code in a theme outside of a hook anyway. But OP asked what circumstances were necessary for parent functions to work, and this is one of them. – Jacob Peattie Nov 28 '18 at 13:42

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