php - Why does empty expect T_PAAMAYIM_NEKUDOTAYIM when a non-variable is given?

itemprop="text">

define('foo',
            'bar');

if (empty(foo)) {

 echo
            'qux';
}

rel="nofollow">http://codepad.org/G1TSK1c6
Parse error: syntax
error, unexpected ')', expecting T_PAAMAYIM_NEKUDOTAYIM on line
4

I know that
empty() only allows variables to be passed as an argument, but
why does it expect a T_PAAMAYIM_NEKUDOTAYIM (i.e. ::) when I
give it a constant?

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class="normal">Answer

The next logical thing the parser
wants is a :: because foo is not a
variable.

if (empty(foo::$bar))
            {
}

Is the
only thing, that works when empty() is not passed a variable.
Your example is evaluated as empty(bar) where the parser
assumes bar to be a class name and now expects a static member
variable.