Type Hinting for Interfaces

<?php 
abstract class Car { 
  protected $model; 
  protected $height;
  
  abstract public function calcTankVolume(); 
}
// Bmw class
class Bmw extends Car {
  // Properties
  protected $rib;
  
  public function __construct($model, $rib, $height) {
    $this -> model = $model;
    $this -> rib = $rib;
    $this -> height = $height;
  }
  
  // Calculate a rectangular tank volume
  public function calcTankVolume() {
    return $this -> rib * $this -> rib * $this -> height; 
  }
}
// Mercedes class
class Mercedes extends Car {
  // Properties 
  protected $radius;
  
  public function __construct($model, $radius, $height) {
    $this ->model = $model;
    $this -> radius = $radius;
    $this -> height = $height;
  }
  
  // Calculates the volume of cylinder
  public function calcTankVolume() {
    return $this -> radius * $this -> radius * pi() * $this -> height;
  } 
}
// Type hinting ensures that the function gets only objects 
// that belong to the Car interface
function calcTankPrice(Car $car, $pricePerGallon)
{
  return $car -> calcTankVolume() * 0.0043290 * $pricePerGallon . "$"; 
}
// Bmw object
$bmw1 = new Bmw('62182791', 14, 21); 
echo "Full rectangular shaped gas tank costs " . calcTankPrice($bmw1, 3);
echo "\n";
// mercedes Object
$mercedes1 = new Mercedes('12189796', 7, 28); 
echo "Full cylindrical shaped gas tank costs " . calcTankPrice($mercedes1, 3);
?>