[php-langspec][PR #234] - Define encoding of PHP scripts (Fix #233)

You can view the Pull Request on github:
https://github.com/php/php-langspec/pull/234

Comment by IMSoP:
Per my most recent comment on the Issue, I think a reasonable wording would be something like:

> PHP scripts are parsed as a series of 8-bit bytes, rather than code points from Unicode or any other character repertoire. Within this specification, bytes are represented by their ASCII interpretations where these are printable characters.