Yip, works now :-)

Yep, works fine now, thanks :-)

Error: Call to undefined function to2D()

When trying with PHP. This call definitely comes from the tests, I have no such function call.

I agree, I was confused by it too; assumed it to be sorted by the alphabetical character only, and not the whole substring.

But apart from that, looking at other solutions… wtf, that complexity of the solutions.

Each time I try to submit, I get one of the following error messages (I definitely have no $expected_result variable in my own code):

testRandomCheck
✘ Undefined variable: expected_result
Completed in 0.29ms

testRandomCheck
✘ Failed asserting that false is identical to 0.
Completed in 0.40ms

There seems to be something broken in testRandomCheck; how would false ever be expected?