Exotic names for methods, constants, variables and fields - Bug or Feature?

after some confusion in the comments to

• Is it safe to have 1 letter class names in PHP, e.g A, B, C

I thought I make into a question. According to the PHP manual, a valid class name should match against [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*. But apparently, this is not enforced, nor does it apply for anything else:

define('π', pi());
var_dump(π);

class ␀ {
    private $␀ = TRUE;
    public function ␀()
    {
        return $this->␀;
    }
}

$␀ = new ␀;
var_dump($␀ );
var_dump($␀->␀());

works fine (even though my IDE cannot show ␀). Can some erudite person clear this up for me? Can we use any Unicode? And if so, since when? Not that I would actually want to use anything but A-Za-z_ but I'm curious.

Clarification: I am not after a Regex to validate class names, nor do I know if PHP internally uses the Regex it suggests in the manual. The thing that confused me (and apparently the other guys in the linked question) is why things like $☂ = 1 can be used in PHP at all. PHP6 was suppposed to be the Unicode release but PHP6 is in hiatus. But if there is no Unicode support, why can I do this then?