Using a filter with multiple parameters and $this

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There are a lot of fundamental mistakes here that suggest a misunderstanding of what filters are, and how they work.

Filters are a way for you to intercept and modify a value. For example:

add_filter( 'my_number', function( $value ) {
 return $value + 1;
} );
$stuff = apply_filters( 'my_number' , 5 );
echo $stuff; // outputs 6

Here, we have the number 5, and we give other code the opportunity to modify it. Notice the filter that was added that adds 1 to every value. The value we wanted to modify was passed in as the first value, and is the only thing absolutely necessary. We then modified it and returned the new value.

A filter can pass additional parameters, but these are just extra information. They can’t be modified and are entirely optional.

$final_result = apply_filters( 'filter name', $thing_to_be_filtered, $other_parameters_that_are_all_optional, etc.. );

Your code does none of that:

  • Your code returns no value to act as a replacement, so PHP will return null or an empty value, aka return '';
  • Your code calls echo with no output buffers, so the output would get sent straight to the browser without going through the proper process
  • for some reason you ignored the first value and renamed it to $object_id, the second value, so all your parameters are off by one. The value being filtered is the HTML, which is why it calls ob_get_clean() to grab the value of the HTML sent to output. That’s what’s being filtered, not the order ID.

As for the filter you’re working with, you have 2 options for changing the anchor tag:

  • modify the string passed as the first value
  • Ignore the value to be filtered and generate a new one from scratch

It looks like you tried to do the second, but got the function arguments wrong, and didn’t return anything. I’d suggest doing the latter option as modifying a HTML string is not easy or reliable.

Finally, there’s a bug here:

            $formatted = $this->get_formatted_tracking_item( $order_id, $tracking_item );

$this can only be used inside objects, but this is a standard PHP function, not an object. This line will cause a PHP fatal error. You will need to find an alternative method of generating this link via WooCommerce support. This kind of bug implies that there are some gaps in the basic fundamentals of how functions and objects work in PHP/general programming, a refresher should make all of this a lot clearer/easier

The same mistake is also made here:

$tracking_items = $this->get_tracking_items( $order_id );

However, since $tracking_items is passed by the filter, this line is unnecessary, just reuse the tracking items it’s already fetched.