The syntax could be improved by allowing you to omit the (...) part entirely for single argument functions and using currying for functions that need additional arguments. So you would end up with something like:

  $result = $arr
      |> select_column('tags')         // Gets an array of arrays
      |> fn($x) => array_merge(...$x)  // Flatten into one big array
      |> array_unique                  // Remove duplicates
      |> array_value                   // Reindex the array.