Call function from main WordPress theme in a plugin - php

Question

I have a function in my theme functions.php file which returns a value:
function my_theme_function() {
return "100";
}
Anywhere in my theme templates I can simply do this...
echo my_theme_function()
...and I see the number 100 on the page. That's cool.
But in my plugin I would have expected to be able do also get access to this function by echoing my_theme_function() but instead I get a 'call to undefined function' error.
The strangest part is I'm certain this was working a couple of days ago, but I've not touched the code since. I suspect some WordPress shenanigans, but I don't know why or how to get around this.

Answer 1

The reason you may take this result can be the order in which the theme and the plugins are loaded.
For example, your plugin can get loaded before the theme, and obviously, in this case, the function it is not available in your plugin source code.
The solution to this issue are the WordPress Hooks. I don't know what is your plugin code style, but you can bootstrap your plugin in the init hook or even better in the after_setup_theme.
So for example, let's say, you need your plugin should run once your theme is loaded by the WordPress. You can use the following code to do so:
function my_theme_is_loaded() {
// Bootstrap your plugin here
// OR
// try to run your function this way:
if ( function_exists( 'my_theme_function' ) ) {
my_theme_function();
}
}
// You can also try replace the `after_setup_theme` with the
// `init`. I guess it could work in both ways, but whilw your
// plugin rely on the theme code, the following is best option.
add_action( 'after_setup_theme', 'my_theme_is_loaded' );
What the above code does, is like you say to your plugin, wait until the theme is totally loaded, and then try to run my plugin code that rely on the theme code.
And of course, I suggest either wrap your theme function in a plugin function like that:
// This way, your plugin will continue running even if you remove
// your theme, or by mistake your rename the function in the theme
// or even if you totally decide to remove the function at all in the
// side of the theme.
function function_from_theme() {
if ( function_exists( 'my_theme_function' ) ) {
return my_theme_function();
} else {
return 0; // Or a value that is suitable with what you need in your plugin.
}
}
This is going to protect your site against theme de-activation or theme change. In this cases, you are going to have a plugin looking for a function in your theme, and while you change the theme or deactivate your theme, your plugin will break your site.