callable.php

<?php
function foo(callable $bar)
{
    $bar("nu?\n");
}

foo(function($arg) { echo "Hello world!\n" . $arg;});

function baz($arg)
{
    echo "Hello baz\n" . $arg;
}

foo('baz');

class Yo
{
    function hey($arg)
    {
        echo "Yo!\n" . $arg;
    }

    static function ho($arg)
    {
        echo "Yo?\n" . $arg;
    }

    function __call($method, $args)
    {
        echo "Ha jo\n", $args[0];
    }

    static function __callStatic($method, $args)
    {
        echo "Ha noi!\n" . $args[0];
    }
}

foo(['Yo', 'ho']);

foo(['Yo','other']);

$yo = new Yo();

foo([$yo, 'hey']);

foo([$yo, 'noi']);

results.txt

Hello world!
nu?
Hello baz
nu?
Yo?
nu?
Ha noi!
nu?
Yo!
nu?
Ha jo
nu?