gistfile1.php

<?hh

type Example = (function(): string);

function main() {

        $callback = function() {
                return 'foo';
        };

        echo acceptCallable($callback);
        echo acceptCallable(['Foo', 'printBar']);

}


class Foo {
        function printBar(): string {
                return 'bar';
        }
}

function acceptCallable(callable $callback): mixed {
        return $callback();
}

main();