Different conditions = different Problem. Instead of writing here, which works fine with the kata conditions, it would be better if you post in some of those other solutions saying "it works for more than 8 digits too". Cheers.

"... for an observed PIN with a length of 1 to 8 digits"

Thanks for mentioning that.
This has to do with trying to read entries that do not exist. (Looking for the button above a button on the most upper row of buttons does not make sence right??)

I did not encounter this problem at the time I submitted this, as this was allowed by PHP and would result in a 'null' value (hence the check if the result is not null).

Replacing the checks with a more PHP-friendly way should do the trick, so line 8 would be:
if($j > 0) $vals[] = $notepad[$i][$j-1]
This is less pretty and satisfying to look at, but oh well...

oh, yes