From the Kata Description:

Preloaded for you is a function say_list which receives an array as its only argument and has the following behaviour:

say_list(["Apple"]) // => "Apple"
say_list(["Apple", "Orange"]) // => "Apple or Orange"
say_list(["Apple", "Banana", "Dragonfruit", "Kiwi"]) // "Apple, Banana, Dragonfruit or Kiwi"

You may find this function useful when implementing one of the methods of one of the classes in this Kata.

thank you, if it wasn't for your comment, I'd have given up on this kata!

Hmm, where is saylist() defined? I can't find it?

greet_extraterrestrials as required by the Kata description, should be "... Therefore, define a static class method called greet_extraterrestrials which accepts an argument...", but yours doesn't look like static to me.

Also this means, that the test should be tested in the tests with ReflectionMethod::isStatic may be?

Updated :-)

It should be noted in the instructions that each floor have only one staircase. Current wording is a bit ambiguous.

It's very dense and a bit hard to understand.
It's clever for sure!
I wouldn't hold it as an example of "best practice" though, since the code should be readable even if it's a bit longer. It'll be easier to mantain in the long run.

In the instructions it should be mentioned that all amount values in the rows are expected to be floats rounded up to 2nd digits after the dot.

For example:

003 Hardware 93.5

is expected to become:

003 Hardware 93.50

This is not mentioned, so it's discovered after the failed tests on the server and it can lead to refactoring.