Which DokuWiki version do you have?
I searched on my file for "class DokuWiki_Plugin", and I can only find it in the Plugin.php, I don't know why the error occurs.
Fatal error: Cannot declare class DokuWiki_Plugin, because the name is already in use in /Users/yanbingong/Sites/dokuwiki/inc/plugin.php on line 9
Your wiki should only ever have one of those files:
inc/plugin.php (Pre-Greebo)
inc/Plugin.php (Greebo and later)
If you have both, then there is something wrong.