Assumes you have root on the Linux server running your WordPress sites (including via using sudo), it's one that uses systemd, and you use php-fpm to provide PHP rather than having it in every webserver instance.

Replace 8.4 with your version as necessary.

When the PHP has been patched, you can do systemctl start php8.4-fpm.service to get everything back running. In the meantime, visitors see a 503 'system unavailable' error.