Passing function as an argument of another function in PHP

Question

I want to pass function (not the result) as an argument of another function. Let's say I have

function double_fn($fn)

and i want to run given function ($fn) two times. Is there such mechanism in PHP? I know you in python function is a kind of variable but is PHP similar?

@edit is there similar way to use methods?

function double_fn(parrot::sing())

Answer 1

Since 5.3 (note) you do this with closures quite naturally:

function double_fn(callable $fn)
{
    $fn();
    $fn();
}

double_fn(function() {
    echo "hi";
});

(note) type hint only since 5.4

The callable type can be a few things:

1. a string comprising the function name to call,
2. a closure (as above)
3. an array comprising class (or instance) and method to call

Examples of the above is explained in the manual.

Update

edit is there similar way to use methods?

function double_fn(parrot::sing())

Doing that will pass the result of parrot::sing() into the function; to pass a "reference" to the static method you would need to use either:

double_fn('parrot::sing');
double_fn(['parrot', 'sing']);

Answer 2

It's... a bit different. Pass the function name as a string.

function foo()
{
  echo "foobar\n";
}

function double_fn($fn)
{
  $fn();
  $fn();
}

double_fn('foo');