6
My guess is that your PHP 5 is not configured to handle long superglobals ($HTTP_POST_VARS etc.) - it is disabled by default in PHP 5 and although I have looked for it, I have not been able to locate that setting in php.ini of PHP 5.
Xoops 2.0.9 uses only $_POST, $_GET etc. so that may be the reason why you can install that version but not 2.0.7.3
2.0.7.3 may install, but will not work properly on PHP 5 because of the new, native method clone() which clashes with the clone() method in the XoopsObject class.