This library provides a JSON decoder and encoder for PHP, based on the native PHP json_* functions, but with full Unicode support, following RFC 7159. The native PHP functions only support JSON ...

I tried to closely follow the algorithms in the spec, as a way of testing them, but when it came to: "6. Increase utf-8 bytes seen by one and set utf-8 code point to utf-8 code point + (byte − 0x80) ...