modelling-1.php

<?php

class Pelanggan
{

}

class Roti
{
	private $jumlah;
	
	private $harga = 2000;
	
	public function __construct(int $jumlah)
	{
		$this->jumlah = $jumlah;
	}
	
	public function beli() : Invoice
	{
		return Invoice::create($this);
	}
	
	public function jumlah() : int
	{
		return (int) $this->jumlah;
	}
	
	public function harga() : int
	{
		return (int) $this->harga;
	}
}

class Invoice
{
	private $total;
	
	private $diskon = 10;
	
	public static function create(Roti $roti)
	{
		$object = new self();
		
		$object->total = $roti->harga() * $roti->jumlah();
		if ($roti->jumlah() > 5) {
			$object->total -= $object->total * ($object->diskon/100);
		}
		
		return $object;
	}
	
	public function total() : int
	{
		return $this->total;
	}
}

$pelanggan = new Pelanggan();

// Skenario 2: Beli 2 roti total invoice harusnya 4000 tidak mendapat diskon
$roti = new Roti(2);
$invoice = $roti->beli();

if ($invoice->total() === 4000) {
	echo "Skenario 1 berhasil". PHP_EOL;
}

// Skenario 2: Beli 10 roti total invoice harusnya 18000 (sudah didiskon 10%) jika beli roti > 5
$roti = new Roti(10);
$invoice = $roti->beli();

if ($invoice->total() === 18000) {
	echo "Skenario 2 berhasil". PHP_EOL;
}