The PHP code incorrectly assumed that the digits are single-byte in
UTF-8, which is never the case (except for 0-9).
The JS code worked correctly because it uses UTF-16 strings, so the
bug would only affect non-BMP digits there. This was noted in a TODO
comment, but we overlooked it when reimplementing in PHP.
Instead of a string of 10 characters, use an array of 10
single-character strings.
Bug: T261706
Change-Id: Ic5421382474c88f003424799c53ff473d99cce92