When does remove_filter not work?

Question

There are reports that remove_filter does not work under some circumstances, and that we should provide alternatives. However, the article does not really make it clear under which circumstances this happens.

I am looking for code examples that will break remove_filter, with WordPress and PHP version, and eventually other relevant info, provided. I think the following snippet should be a useable template:

<?php
include('wp-load.php');

function filtertest_function($value)
{
    return 'Filtered';
}

var_dump(apply_filters('filtertest', 'Original value'));
add_filter('filtertest', 'filtertest_function');
var_dump(apply_filters('filtertest', 'Original value'));
remove_filter('filtertest', 'filtertest_function');
var_dump(apply_filters('filtertest', 'Original value'));

This will return the expected:

string(14) "Original value"
string(8) "Filtered"
string(14) "Original value"

I believe the error comes up in some cases where the filter is part of a class, or multiple filters are used. I understand _wp_filter_build_unique_id is involved. Please point out where in that code the source of the error is.

Answer 1

Well this is somehow a very specific topic that is bound to a Wordpress development issue. I strongly suggest you to keep track of the trac ticket if you liked my article. That is the best thing you can do I assume for finding out when the problems come into play as well how to circumvent them technically (if you don't like the don't use remove_filter()-answer).

Take it from a theoretic standpoint: It's just that the used datatypes are not strictly dealt with to ensure the same functionality on all possible values (f(n) != f(n)). In short: a broken design.

Does this mean it will always break in practise? - No! It's just that it can happen sometimes. And then you're trapped when you need to rely on remove_filter().

A better suggestion might be this one: If you develop plugins that make use of hooks as class methods, ensure that the plugin get executed on installations with PHP 5.2 / 5.3.

Please keep the technical discussion in the trac ticket. And if you're seriously interested, please help to fix the shortcomings of the current design.

Answer 2

As Denis commented on Hakre's blog, this is a very basic example of the issue.

function run_me_once() {
    remove_action( 'foobar', 'run_me_once' );
    echo 'test....';
}
add_action( 'foobar', 'run_me_once' );

function run_me2() {
    echo 'test2....';
}
add_action( 'foobar', 'run_me2', 11 );

echo '';
do_action( 'foobar' ); // test....
echo '';
echo '';
do_action( 'foobar' ); // test2....
echo '';

Before running that code though, ask yourself, what would you expect the result to be. Once you've decided what the result should be, run it and see what happens.