PHP OOP with Singleton

Bird.php

<?php 

ini_set('display_startup_errors', 1);
ini_set('display_errors', 1);
error_reporting(-1);

interface Bird{
    public function layEgg($egg_name);
}

class Chicken implements Bird{

    private $name = '';
    private $stamp = '';

    private $eggs = [];
    private static $instances = [];

    public function __construct($name){
        $this->name = $name;
        $this->stamp = date('YmdHis');
    }

    public function layEgg($egg_name)
    {
        $this->eggs[] = new Egg('Chicken', $egg_name, date('YmdHis'));
    }

    public function getEggs()
    {
        return $this->eggs;
    }

    // singleton
    public static function getInstance($name)
    {
        $cls = static::class;
        if (!isset(self::$instances[$cls])) {
            self::$instances[$cls] = new static($name);
        }

        return self::$instances[$cls];
    }
}

class Egg{

    private $type = '';
    private $name = '';
    private $stamp = '';

    public function __construct($type, $name, $stamp){
        $this->type = $type;
        $this->name = $name;
        $this->stamp = $stamp;
    }

    public function hatch()
    {
        // using singleton
        // $new_chicken = Chicken::getInstance("chicken hatched from ".$this->name);

        // using traditional creation
        $new_chicken = new $this->type($this->type." hatched from ".$this->name);

        return $new_chicken;
    }
}

$chicken = new Chicken('Chicken name 1');
for($ii=1;$ii<=2;$ii++){
    $chicken->layEgg("child egg ".$ii);
}
$eggs = $chicken->getEggs();
$childs = [];
foreach ($eggs as $egg) {
    $childs[] = $egg->hatch();
}

// using singleton
// $chicken2 = Chicken::getInstance('Chicken name 2');

// using traditional creation
$chicken2 = new Chicken('Chicken name 2');


echo '<pre>';
var_dump($chicken);
var_dump($childs);
var_dump($chicken2);
echo '</pre>';

?>