The Installation Manual states that Testlink should work with PHP 5.x.x (preferably 5.2). I have found that PHP 5.0.5 does not include a property_exists function that is used by login.php. This causes the login to fail. property_exists was added to PHP in 5.1.0
Thank you for your information. It seems that we must better take care PHP compatibility. The information that we support PHP 5.x is not consistent with code of RC1.