empty() seems to always return TRUE when testing an object
property from inside a method of the object. it works OK 
on regular variables, or when testing the property externally.

test code:
<?php
class test
{
    function test()
    {
        $this->x = NULL;
        print "<li>this->x={$this->x}\n";
        print "<li>is this->x empty? ".(empty($this->x) ? "yes" : "no")."\n";
        $this->x = 'i am not empty';
        print "<li>this->x={$this->x}\n";
        print "<li>is this->x empty? ".(empty($this->x) ? "yes" : "no")."\n";
        $x = NULL;
        print "<li>x={$x}\n";
        print "<li>is x empty? ".(empty($x) ? "yes" : "no")."\n";
        $x = 'i am not empty';
        print "<li>x={$x}\n";
        print "<li>is x empty? ".(empty($x) ? "yes" : "no")."\n";
    }
}
$x = new test;
?>

output:
<li>this->x=
<li>is this->x empty? yes
<li>this->x=i am not empty
<li>is this->x empty? yes
<li>x=
<li>is x empty? yes
<li>x=i am not empty
<li>is x empty? no