Later versions of PHP have the íregister_globalsí set to off. That means that form fields, rather than automatically being set to a variable, must be accessed by the script as í$HTTP_POST_VARS[site_name_in]í ($site_name_in being the variable that triggers the next step in the installation process.) If you can change that setting, there should be no problem.
Try placing the following code at the very beginning (just after the <?php) of the install.php script, and see if it solves the problem:
while (list ($key, $val) = each ($HTTP_POST_VARS))
Note that the reason the PHP people decided to make íregister_globalsí default to off is because of security reasons. I donít think bypassing it in this way would be any sort of security threat since the installation script is not meant to be run by anyone but the site owner. But, the same problem may exist in the main script, which would be solved in the same way. I still donít think there would be any sort of security risk since the script never really deals with any sort of sensitive areas -- it simply pulls the verses from the database.