Question

When do closures have parameters (or how do closures with parameters work)? I know that use() is used to import variables outside the anonymous function, but what about the parameter(s) of the closure itself?

Answer 1

An example of closures with parameters is currying:

function greeter($greeting)
{
  return function($whom) use ($greeting) {
    // greeting is the closed over variable
    return "$greeting $whom";
  };
}

$hello_greeter = greeter('hello');

echo $hello_greeter('world'); // will print 'hello world';

The greeter function will return a “half-implemented” function that will always start with the same greeting, followed by whatever is passed into it (e.g. the person to greet).