Comment by qwertox

I don't see how this is hard to reason about, assuming this is the resulting code when using variables:

  $tags        = ...array_column($arr, 'tags');
  $merged_tags = array_merge($tags);
  $unique_tags = array_unique($merged_tags);
  $tag_values  = array_values($unique_tags);

It also makes it easier to inspect the values after each step.