Most likely it is because you are mixing tabs and spaces for indentation: some of your lines of code start with spaces, other with tabs. The number of spaces a tabulation is equal to is not always the same on all renderers and code editors.

Fixed.

That's not the test you're failing. You're failing a test with 2 empty arrays.

Consider adding something like this at the start of function:

echo "\n--------------\n";
var_dump($a1);
var_dump($a2);

Not a kata issue.