When you apply the principle of Domain Driven Design (DDD) to your code, you start to notice all the small “rules” of the domain. Rules like “A name must not consist of whitespace, must have its whitespace trimmed on both ends and cannot be empty." Or “A donation amount must be greater than zero." Or “A book must have at least one page." It’s important that you can’t create value objects that violate those rules.
Test-driven driven development in PHP can become a pain when you’re dealing with the file system. The builtin functions like stat, getfilemtime, fopen and fgets assume the existence of actual files. Until now, I assumed you’d have to add a library like FileFetcher, Flysystem, Gaufrette or vfsSystem to your dependencies. While those libraries are nice, they are additional dependencies and some add additional capabilities like caching or providing a unified interface to cloud storage.