Function names cannot be keywords

[mattacosta]

Summary
A function name must be a T_STRING token (or whatever its equivalent is in this parser).

Sample code:
function empty() {}

Expected
Parse error: syntax error, unexpected 'empty' (T_EMPTY), expecting '('

Actual
No error.

[jens1o]

You may not redeclare language constructs.

[TysonAndre]

https://3v4l.org/90mgr

It is still a parse error which should be detected. To contrast, function strlen($s) {} would not be a parse error, but evaluation would result in a PHP error at runtime.

Also something to note: empty is a valid name for a class method in PHP 7

[felixfbecker]

@jens1o empty is a language construct, not a function

[jens1o]

Nice catch :)