from collection import defaultdict

DefaultDict.php

<?php
class DefaultDict extends ArrayObject
{
    private $ctor;

    public function __construct($ctor, array $ary = [])
    {
        if (is_array($ctor)) {
            list($class, $method) = $ctor;
            if (is_object($class)) {
                $this->ctor = function () use ($class, $method) {
                    return $class->$method();
                };
            } else {
                $this->ctor = function () use ($class, $method) {
                    return $class::$method();
                };
            }
        } else {
            $this->ctor = $ctor;
        }
        parent::__construct($ary);
    }

    public function offsetExists($key)
    {
        return true;
    }

    public function offsetGet($key)
    {
        if (parent::offsetExists($key)) {
            return parent::offsetGet($key);
        }
        $ctor = $this->ctor;
        $value = $ctor();
        $this->offsetSet($key, $value);
        return $value;
    }
}

wc.php

<?php
require 'DefaultDict.php';

error_reporting(E_ALL);

function wc($s, DefaultDict $d) {
    foreach (str_split($s) as $c) {
        $d[$c] += 1;
    }
    print_r((array)$d);
}

class X {
    public function int() { return 0; }
    public static function integer() { return 0; }
}

$s = 'abracadabra';
wc($s, new DefaultDict(function(){ return 0; }));
wc($s, new DefaultDict(['X', 'integer']));
wc($s, new DefaultDict([new X, 'int']));