Online PHP editor | output for a63MU

<?php abstract class Price { protected $value; public function __construct($value) { $this->value = $value; } // Duck typing means this declaration is actually optional abstract protected function getRawValue(); public function getFormattedValue() { // Calls a protected method of whichever child class is $this return number_format($this->getRawValue(), 2); } public function add(Price $other) { // Calls a protected method of whichever child class is $other $this->value += $other->getRawValue(); } } class TaxFreePrice extends Price { protected function getRawValue() { return $this->value; } } class PriceWithTax extends Price{ private const TAX_RATE=1.2; protected function getRawValue() { return $this->value * self::TAX_RATE; } } $a = new TaxFreePrice(10); $b = new PriceWithTax(10); echo $a->getFormattedValue(), PHP_EOL; echo $b->getFormattedValue(), PHP_EOL; // Here, an instance of TaxFreePrice will access a protected member of PriceWithTax $a->add($b); echo $a->getFormattedValue(), PHP_EOL;