[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 nikic:
I've reverted this again in 9d99cf029b84f40b99bdab0224c79dd89c09e41a. I didn't notice that this changed the grammar file -- this is an auto-generated file and not an appropriate place for any spec text.

While looking for a good place to put this text instead I found https://github.com/php/php-langspec/blob/master/spec/09-lexical-structure.md#scripts, which requires that at least UTF-8 is supported and defines a dubious "transformation" step.

This is the section that needs to be adjusted, but probably in not quite the same way as just the added text in this PR.