Thanks for the suggestion. It looks like there is something more problematic happening. I did find an _config.php file in the mysite folder, so I don't think that is the problem. Even still I've replaced it with the version that you suggested (of course I edited it to contain the details of the mysql DB for my web hosting account). Still I get the same fatal error as before.
Could this issue be something related to Omnis.com's settings on mod_rewrite perhaps? There's at least one other person who has mentioned an issue with Omnis, but there wasn't a resolution:
I've also manually created a .htaccess file in the root of my Silverstripe installation with a variety of rewrite rules (I don't understand mod_rewrite very well, so this was just a guess too). That doesn't help either. If it matters, I've put in extra echo commands through the chain of calls to see exactly where the install.php stops. I find that I can get as far as :
$conn = new $dbClass($databaseConfig);
in the function connect() in the file sapphire/core/model/DB.php. Apparently that's when the error occurs:
Fatal error: Class MySQLQuery cannot extend from interface Query in /mywebserver/www/silverstripe/sapphire/core/model/MySQLDatabase.php on line 452
I am not a knowledgeable enough in PHP to tell how to debug this issue further. Thoughts?