gistfile1.php

<?php

class Animal {}

class Cat extends Animal {}
class Dog extends Animal {}

class Manul {}

$c = new Cat;
$d = new Dog;

// magic begins
use Codegyre\Implicit;

Implicit::setValue($cat); // set implicit value

function pogladit($animal = null) {
  $animal = Implicit::get('Animal', $animal); // get implicit value if not set explicitly
  // ...
}

pogladit(); // implicit cat

pogladit($c); // explicit cat
pogladit($d); // explicit dog


// supermagic

$fromCatToManul = function (Cat $cat) {
  return new Manul;
};

Implicit::setConversion('Cat', 'Manul', fromCatToManul);

function pogladKotaSuka($manul = null) {
  $manul = Implicit::get('Manul', $manul);
  // ...
}

pogladKotaSuka($c); // Cat will be implicitly converted to Manul