It depends on the SAPI. The php-fpm approach is a pool of processes that are reused for multiple requests. Virtually all of the state visible to plain PHP code is reset between requests, however some interpreter/runtime state is kept, e.g. in-memory bytecode cache, initialized extensions, some persistent network connections, etc.

In terms of memory management, PHP has both reference counting and garbage collection. Internally, an arena per request is used[0], so leaking memory over long periods of time is fairly rare and usually limited to native extensions.

[0]: https://www.phpinternalsbook.com/php7/memory_management/zend...