19-interpreter.php

<?php

interface Converter
{
    public function show();
}

// === A Simple Converter ===
// That convert Gallon to Litre
class GallonToLitre implements Converter
{
    private $gallon;

    public function __construct($gallon)
    {
        $this->gallon = $gallon;
    }

    public function show()
    {
        return round($this->gallon * 3.79);
    }
}

// === A Simple Converter ===
// That convert Mile to Kilometer
class MileToKilometer implements Converter
{
    private $mile;

    public function __construct($mile)
    {
        $this->mile = $mile;
    }

    public function show()
    {
        return round($this->mile * 1.6);
    }
}

// === An Intepreter ===
// That interpret to Kilometer per Litre by using existing converters.
// Consider this as structuring a sentence using grammers (converters).
class MpgToKml implements Converter
{
    private $g2l;
    private $m2k;

    public function __construct(Converter $g2l, Converter $m2k)
    {
        $this->g2l = $g2l;
        $this->m2k = $m2k;
    }

    public function show()
    {
        echo  $this->g2l->show() . "l/" . $this->m2k->show() . "k\n";
    }
}

// ---

$mpg2kml = new MpgToKml(new GallonToLitre(1), new MileToKilometer(20));
$mpg2kml->show();
// Output: 4l/32k