ePHPm can serve a directory full of sites out of one process. Point sites_dir at a folder and every subdirectory is a virtual host. The Host header decides which one you get. That’s how the PR preview environment runs. Every pull request gets a subdomain, and the code behind it is whatever somebody pushed. One process holding many tenants means ePHPm enforces every per-tenant boundary itself, in-process. There’s no container wall between site A and site B unless you add one.

php-fpm has a property nobody thinks about until they leave it. When a worker segfaults, you lose one request. The master forks a new child and life goes on. ePHPm runs every tenant’s PHP as threads inside one process. That’s where the memory and latency wins come from. It also means one tenant’s segfault takes down every other tenant on the box.

In multi-tenant mode ePHPm serves every site in sites_dir from one process. Every tenant’s PHP runs as the same OS user on the same pool of ZTS threads. A thread that just served blog.example will serve shop.example a few milliseconds later. That’s the density win. It also means the kernel has no idea which tenant is behind a given connect(). As far as Linux is concerned it’s all ephpm-web.