Modern PHP applications can avoid the continuous bootstrapping on each request from client browsers by creating a daemon.
Old school PHP software like WordPress with it’s famous the_loop() that relies heavily on global variables and is thus hard or impossible to modify to work this way. All applications written in PHP can access the superglobal values and there is no way of sandboxing them.
This is why PSR-7 and other request agnostic methods are not feasible to use with WordPress applications. This is where the technical debt of WordPress might start to drag it down as a non-viable option to applications with a more sophisticated architecture.
- Refactoring WordPress core from using globals
- Long running Drupal 8 HTTP web workers
- Things to consider when developing an application with PHPFastCGI
- Running PHP daemons with PHP-PM or PHPFastCGI