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Date: Fri, 25 Mar 2016 14:20:13 +0000
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Subject: Why is 0x7F permitted in PHP identifiers?
From: ajf@ajf.me (Andrea Faulds)

Hi everyone,

Identifiers in PHP source code (including variables names with $) 
conform to the regex /[_a-zA-Z\x7F-\xFF][_0-9a-zA-Z\x7F-\xFF]*/. Most of 
this regex is pretty standard: it allows alphanumeric ASCII characters 
and underscores, plus any character with the 8th bit set (presumably to 
allow any extension of ASCII, such as Latin-1 or UTF-8, to be used).

But there's one part of this I find rather curious: why is \x7F 
included? It's not a high-byte/8-bit character, it's a 7-bit ASCII 
character, and a control character at that. Unless there's some ASCII 
extension which reuses that value as a printing character, I assume it 
must have been a mistake to include this character. As a control 
character, it is invisible and difficult to type, and it might do weird 
things in some terminal emulators. I can't see the value in permitting 
it within an identifier.

I've done a little bit of looking around, and I can't find an important 
ASCII extension which changes what 0x7F does. Given that, I assume it 
was simply a mistake. But one of you might be able to enlighten me 
otherwise.

I've filed a bug report, and made a patch to fix this in php-src and 
php-langspec master:

https://bugs.php.net/bug.php?id=71897

Thanks!
-- 
Andrea Faulds
https://ajf.me/