A PHP upgrade can take a working WordPress site down in seconds. Hosts retire older PHP versions for good reasons: security, performance, and long-term compatibility. The trouble starts when outdated plugins, themes, or custom code meet that newer runtime.
Know what you are running
Write down the current PHP version and the target version. Moving from PHP 7.4 to PHP 8.x can introduce compatibility changes that never appeared as warnings on the old version. A fatal error on PHP 8 may have been a silent notice before.
Check plugin and theme compatibility
Review vendor documentation, tested PHP versions, recent updates, and known issues. Abandoned components deserve extra suspicion. If the author vanished two years ago, a PHP bump is often the moment that code finally fatals.
Test on staging first
Clone the site, switch PHP there, and test the homepage, login, search, forms, and for stores the cart and checkout. Enable WP_DEBUG and WP_DEBUG_LOG on staging so warnings and fatals land in a log. Keep error display off and the log private.
Have a rollback plan
Take a fresh backup and confirm the host still allows the previous PHP version. Switching PHP on a live site through the hosting panel without a test, then learning about the problem from customer complaints, is a pattern I see often.
The real fix is not staying behind forever
Old PHP is a security problem. The durable fix is updating or replacing the code that cannot keep up, then moving forward on a version the host will keep supporting. If you want help with a site that broke after a PHP change, see WordPress emergency fixes.