Do you know what empty(false) evaluates to? or empty('0')? How about ('-1' == true)?
Well see yourself :-)
This test was created based on php 4.0.5, and is updated for php 4.2.2.
All this has been proofed on Win2k using PHP 4.2.2 using the PhpUnit test class "PhpSyntax_PhpUnit.class.php".
It is included in the download package.
Once you're done you might want to check the PHP Cheat Sheet.