A few days ago I installed WordPress 2.7 RC1 on one of my blogs to get a feel for the new interface. One of the awesome new features of WP 2.7 is the built-in upgrade feature, a feature that the very cool WordPress Automatic Upgrade plugin has handled for me up until now.
I was eager to try the new built-in upgrade feature, so when WP 2.7 was officially released I rushed to upgrade the RC1 blog (
Tools -> Upgrade). To my surprise however, I was shown something that did not even remotely look like a success message:
Fatal error: Cannot redeclare class pclzip in /home/homeproj/public_html/wp-admin/includes/class-pclzip.php on line 171
After a little research, I discovered the problem was the WordPress Automatic Upgrade plugin! Both the WPAU plugin and the built-in upgrade functionality of WordPress 2.7 must declare the same
pclzip class. This means you cannot have the WPAU plugin active and use the built-in upgrade functionality of WP 2.7 at the same time.
To fix the problem, you simply need to deactivate the WordPress Automatic Upgrade plugin.
If you don't have the WPAU plugin installed, you can try deactivating all of your plugins to see if any of them are causing a conflict.