How a security vulnerability took down most of the internet

Many websites have been down within the past 24 hours for unplanned maintenance. But what's going on that's lead to these websites shutting down?

No, it's not cyberwar.

PHP, which is used by over 70% of websites (including some of my own), had a security flaw discovered by researchers with the help of AI. The flaw, referred to as PHP2Shell, is a remote code execution (or RCE) exploit. The PHP Foundation was quick to release a patch, but the patch was only for supported versions of PHP, and even then, the vast majority of websites had to be taken down to deploy the patch.

As of posting, some websites I've been tracking are back up, while some are still down to deploy the patch.

As for my websites, all of my affected websites have been patched as of this post.