<?php

abstract class Polygon {
    protected $sides = 0;
    abstract protected function area();
    public function getSidesNum() {
        return $this->sides;
    }
}

class Triangle extends Polygon {
    private $width = 0;
    private $height = 0;
    function __construct($w, $h) {
        /* parent::__construct(); perhaps abstract classes do not have a ctor */
        $this->width = $w;
        $this->height = $h;
        $this->sides = 3;
    }

    public function area() {
        return $this->width * $this->height * 1/2;
    }
}

$rightTri = new Triangle(4,4);
echo "area: {$rightTri->area()}, sides: {$rightTri->getSidesNum()}" . PHP_EOL;