Warning: require_once(__DIR__/admin.php) [function.require-once]: failed to open stream: No such file or directory in /wp-admin/post.php on line 12

Fatal error: require_once() [function.require]: Failed opening required '__DIR__/admin.php' (include_path='.:/:/usr/local/php5/lib/pear') in /wp-admin/post.php on line 12