How to Know What Breaks Before You Upgrade WordPress to PHP 8.4

Every WordPress developer knows the feeling. A host emails: "PHP 7.4 reaches end of life — we're moving you to 8.x." Or you finally want the performance and security of PHP 8.4. You stage it, click upgrade, and… white screen. Somewhere in the 40 plugins and 3 themes on the site, something called a f