Description:
------------
Given a "property" that is really being handled by __get() and __set() functions, you are allowed to use it with empty() and isset() without errors or warnings, but they always report that the property is empty/not-set.

I understand that this may not be a bug, but a "feature"
of PHP 5 - i.e., just the way it works - but if that is true,
please be kind enough to say so explicitly. It is not helpful
to mark bugs as Bogus with comments like "I suggest you read ZEND_CHANGES :)"

If it is to be expected, it might be good to throw the
same kind of error that one would see if trying to use 
empty() on a function call, if that's possible.

This is possibly related to bug #24436.

Reproduce code:
---------------
class foo
{
    private $_x;
    private function __get($p) { return $this->_x; }
    private function __set($p,$v) { $this->_x = $v; }
}
$y = new foo;
$y->x = 12;
if (empty($y->x))
    print "y->x is empty: {$y->x} \n";
else
    print "y->x is not empty: {$y->x} \n";
if (isset($y->x))
    print "y->x is set: {$y->x} \n";
else
    print "y->x is not set: {$y->x} \n";

Expected result:
----------------
y->x is not empty: 12 
y->x is set: 12

Actual result:
--------------
y->x is empty: 12 
y->x is not set: 12