It's true that pipes are more readable, and for many cases they will be the better option, but the example of nested functions just doesn't hold.

That's like saying someone would use this:

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

which is harder to reason about than the nested functions.

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

or

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

It is more readable and better option — you have to parse it from the innermost function to the outermost just to understand what it's doing. With the pipe, it's more straightforward: you read it step by step — do this, then that, then the next — just like how you'd naturally read instructions.

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(...)

vs

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

With pipes you have linear sequence of data transformations. With nested function calls you have to start with innermost function and proceed all the way top the outermost layer.

Because they were already formatted that way to begin with.

The pipe syntax is much more readable than nested function calls when you need additional arguments for intermediate functions. With nested functions it becomes hard to see which functions those arguments belong to even if you try to help it with formatting.

> so they are more memory efficient

They can be. It depends on the language, interpreter, compiler, and whether you do anything with those intermediate variables and the optimiser can get rid of them.

I thought we are talking about PHP8.5:)

Ah, I thought we were talking more generally about PL constructs that let you avoid intermediate variables, apologies :)