Wait, if the error is that "osclass isn't installed", then it that the database / tables exists but are empty or not correctly setup.
Remove your tables/database, remove your config.php file and try to install Osclass again.
If you error is the one on the main post, first don't hijack topic for other things, create a new one or search a proper topic that talks about your issue (and not a different one!), second, specify your PHP version.