Arguuugh! The "encoded via zend" part pretty much sucks!
I'll bet ya anything the problematic variable is used in the zend encoded part of the install.
Just to confirm the issue, you could try running it under mod_php (PHP version 4.4.7, I believe), as described here (make sure and read the comments for the "tip" on how to do this).
In short, create a text file named ".htaccess" in the root directory of your domain (yourdomain.tld) containing the single line:
AddHandler application/x-httpd-php .php
and try running the install again. If the problem is the suspected environment variable, this should allow it to run.
I hate zend / ioncube/ etc. encoding because it makes it hard /impossible to fix stuff like this.
IF you can confirm that this is the problem, maybe you can get the author to "fix" his code so that it does not rely on a given PHP mode.
Purely my opinion here, but this line from their website would probably queer me on using this code:
"The script will skim 1.5% of your traffic per install, if you want to remove the skim you can simply buy a key to migrate your copies to the commercial non skimming version. Small link back to our site is required."
Hey, everyone is entitled to their own business model, but if the script "skims" traffic, who know what else it does in that encoded code?