Why didn't you format the pipes, too?

  $result = $arr
    |> fn($x) => array_column($x, 'tags')
    |> fn($x) => array_merge(...$x)
    |> array_unique(...)
    |> array_values(...)

   array_values(
     array_unique(
       array_merge(
         ...array_column($arr, 'tags')
       )
     )
   );

Because they were already formatted that way to begin with.