To expand on z92's answer: In my case, I have a huge web application including a ton of framework code that's powering a frontend. In the backend, there are scr

To expand on z92's answer: In my case, I have a huge web application including a ton of framework code that's powering a frontend. In the backend, there are scripts processing many millions of rows each day. These scripts depend on the framework being available (for example to do certain tasks in the context of a specific user, or just for configuration settings). It's infinitely more useful to be able to actually rely on that framework in these scripts than having to recreate all the environment (or the needed parts of it) every time - especially as the framework might change and the scripts should continue to work. This makes these scripts prime candidates to be written in the same language as the web frontend and the framework, which, unfortunately, means PHP. By being able to easily spawn a few subprocesses from these scripts, I can make them a lot faster while still being able to rely on the standard application environment to be there for them.