PHP OPCode Cache
How OPCache works https://www.npopov.com/2021/10/13/How-opcache-works.html
PHP Shared OPCode Cache Leaking Data
Two good articles on how to mitigate security issues when running multiple sites under a single master php-fpm process that may lead to sites leaking OPCode cache data.
Further mitigation is to have multiple php-fpm masters for each site.