What is the best way to pass some information from functions.php to a plugin?

I need to give a user the ability to pass a piece of data from their functions.php file to my plugin. (It can be any data, just something that I can check for; it can be a variable's value, or even the fact that a variable was set or a function was defined.)

I tried the suggestion found here:

Pass A Value From Outside To A Plugin Variable

I plugged in that code verbatim:

On your plugin :

 $value = 0;
 $value = apply_filter('get_value_from_function', $value);

Then on functions.php

add_filter('get_value_from_function', 'my_special_value_treatment', 10, 1);
function my_special_value_treatment ($value){
    return 1;

But that results in an error:

Fatal error: Call to undefined function apply_filter() in /pathtomyplugin/my-plugin-name/my-plugin-name.php on line 93

I don't understand why that fails. What is the best way to do this?

Edited with this addition:

After changing it to apply_filters(), this still happens... In my plugin:

$value = 0;
$value = apply_filters('get_value_from_function', $value);
if ($value===0) {
    $test_info = 'not_passed';
} else {
    $test_info = 'passed';
// $test_info is not_passed
  • Is your $value = apply_filters('get_value_from_function', $value); wrapped in a function? How is that function called? Commented Aug 19, 2015 at 18:05

2 Answers 2


Plugins are loaded before the theme which means that your apply_filters won't have any actual callbacks registered to it. Instead, you need to call your apply_filters sometime after the theme has been loaded. Something like this:

/* Your plugin's file: */

add_action( 'init', 'my_lovely_funky_filters' );

function my_lovely_funky_filters() {
    /* Fire our callbacks */
    $value = 0;
    $value = apply_filters( 'get_value_from_function', $value );

    /* Check your $value now */
    /* echo "<pre>{$value}</pre>"; */

/* Your functions.php file */

add_filter( 'get_value_from_function', 'my_special_value_treatment', 10, 1 );
function my_special_value_treatment( $value ) {
    /* A little more interesting */
    return (int)$value + 1;
  • Thanks. This answers the question I asked. Still can't get some other related things to work, but that will have to be a second question!
    – hommealone
    Commented Aug 19, 2015 at 18:50
  • So now the value of $value is indeterminate until the init action fires; I can't seem to access its final value from within the plugin file. How can I retrieve it and use it in my plugin file?
    – hommealone
    Commented Aug 20, 2015 at 0:08

This is the correct method - except the function is apply_filters, not apply_filter. Hence your error.

  • Still no good. I added more details above.
    – hommealone
    Commented Aug 19, 2015 at 17:48

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