[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 Agares:
ASCII uses 7-bits, which in case of 8-bit bytes leaves the highest bit always set to 0. 
Unicode and UTF-8 are not synonyms. Unicode is basically a list of characters (+ specification which defines how they work together, how they are encoded, etc.). UTF-8 is one of unicode encodings, but it's not just "any bytes", but the unicode codepoints encoded in a specific way, in 1-4 bytes. The 0x80 to 0xFF codepoints actually encode as two bytes.