While I appreciate the effort and like the approach in general, in this use case I really would prefer extensions / extension functions (like in Kotlin[1]) or an IEnumerable / iterator approach (like in C#).

  $arr = [
    new Widget(tags: ['a', 'b', 'c']),
    new Widget(tags: ['c', 'd', 'e']),
    new Widget(tags: ['x', 'y', 'a']),
  ];
  
  $result = $arr
      |> fn($x) => array_column($x, 'tags') // Gets an array of arrays
      |> fn($x) => array_merge(...$x)       // Flatten into one big array
      |> array_unique(...)                  // Remove duplicates
      |> array_values(...)                  // Reindex the array.
  ;

feels much more complex than writing

  $result = $arr->column('tags')->flatten()->unique()->values()

having array extension methods for column, flatten, unique and values.

1: https://kotlinlang.org/docs/extensions.html#extension-functi...