SSL error when upgrading a WordPress Multisite Network

When updating to WordPress 4.0, I recently saw a scary-looking error when running the database upgrade on a network with some SSL sites. It triggered when trying to update tables for a site with an SSL certificate.

Warning! Problem updating Your server may not be able to connect to sites running on it. Error message: SSL certificate problem, verify that the CA cert is OK. Details: error:14090086:SSL routines: SSL3_GET_SERVER_CERTIFICATE: certificate verify failed

To be totally honest, I’m not exactly sure why this happens, but I was able to find a way to fix it by telling WordPress to ignore SSL checks when running the database upgrade. Create a file in wpcontent/mu-plugins/ and name it something like network-upgrade-ignore-ssl.php. Put the following in the file:

add_filter('https_ssl_verify', '__return_false');
add_filter('https_local_ssl_verify', '__return_false');

That’s it! You should now be able to get through your upgrade smoothly.