You don't necessarily need to know about the SAPI, but what he says is true. Any environment (Apache, Lighttpd, FPM, CLI, ...) that runs PHP, is built on top of

You don't necessarily need to know about the SAPI, but what he says is true. Any environment (Apache, Lighttpd, FPM, CLI, ...) that runs PHP, is built on top of the SAPI provided by the Zend Engine. That's interesting stuff, but not on the "must know" list.