First-Class Module¶
In OCaml and a few other ML-family languages, a module can itself be treated as an ordinary value: passed as a function argument, returned from a function, stored in a data structure, or chosen at runtime among several implementations of the same module signature, using a construct such as (module M : SIG). This lets an entire package of types and functions be selected or swapped dynamically, the way an object implementing an interface would be in an object-oriented language, but at the granularity of a whole module rather than a single instance.
This differs from ordinary module systems, where a module is only a static, compile-time unit of organization: something that import by name, but never hold, pass around, or compute with as a value.
PHP has no module construct at all, first-class or otherwise; a namespace is a compile-time naming device with no runtime representation and cannot be assigned to a variable, passed as an argument, or chosen dynamically. The closest PHP comes to swapping an entire package of related functionality at runtime is choosing between classes, typically through dependency injection or a factory that returns different objects implementing the same interface.
Related : Module System with Explicit Exports, Namespaces, Interface, Dependency Injection
