Why return true for empty array?
The description says:

If a or b are nil (or null or None), the problem doesn't make sense so return false.
If a or b are empty the result is evident by itself.

"Evident by itself" must mean "the problem doesn't make sense so return false" as well.
Otherwise shouldn't we return true for NULL arrays.

Please make the description clear.

I don't undertand, I have the same problem, if the test is OK then in $a1 should be 190 because of 36100 in $a2. Am I missing something obvious? And what null are you talking about?

Sorry, no bugs in the tests; they are the same in all languages; 8000 guys passed the kata (198 with PHP).