Description:
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Identifiers in PHP source code 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 be a mistake to include this character. As a control character, it is invisible and difficult to type. Worse, if you were to include it within an identifier, it would hide the preceding printing character (because \x7F moves the cursor back one column in terminal emulators).