Explanation for remove_filter used in the below code [closed]

The remove_filter() is necessary to avoid the callback from being called recursively, which would result in a memory issue due to a never-ending function execution.

And that recursion could happen because the callback is hooked to get_post_metadata which is invoked via the get_metadata() function which the callback calls.

So you need to first “unhook”/unregister the callback from the hook and only then you may call get_metadata().

Otherwise, you’d end up with something like:

WP applies the filter (calls apply_filters( 'get_post_metadata', ... ))
- your callback is called
- the callback calls get_metadata()
- WP applies the filter
  - your callback is called
  - the callback calls get_metadata()
  - WP applies the filter
      - your callback is called
      - the callback calls get_metadata()
      - WP applies the filter.....

Or:

your_callback() {
  your_callback() {
    your_callback() {
      your_callback() {
        ..... it never ends ...

So your code is good in that it’s avoiding the unwanted recursion.

However, I noticed that you’re not properly removing the filter:

// You added the filter like so:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// Then in the callback, you're removing it like this:
remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0);

// And later in that callback, the filter is added back:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0, 4);

And the problem is, when removing a filter/action, the priority and number of accepted parameters must match the same values you used when adding the filter:

// If you added the filter like so:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// Then use the same parameters when removing the filter:
remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// And not like this — priority should be -1 and this is missing the fourth parameter:
remove_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0);

And if you initially added the filter like so, then you should also add it back inside the callback the same way — using the same parameters:

// Like this:
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), -1, 4);

// Not this: (priority doesn't match)
add_filter('get_post_metadata', array(WPKWP::CLASS_NAME, 'get_post_metadata'), 0, 4);

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