Create and Using an Interface in PHP

<?php/*from w w w . j  a  va 2  s.  co  m*/

interface Sellable {
  public function addStock( $numItems );
  public function sellItem();
  public function getStockLevel();
}

class Tablet implements Sellable {
  private $_screenSize;
  private $_stockLevel;

  public function getScreenSize() {
    return $this->_screenSize;
  }

  public function setScreenSize( $screenSize ) {
    $this->_screenSize = $screenSize;
  }

  public function addStock( $numItems ) {
    $this->_stockLevel += $numItems;
  }

  public function sellItem() {
    if ( $this->_stockLevel > 0 ) {
      $this->_stockLevel--;
      return true;
    } else {
      return false;
    }
  }

  public function getStockLevel() {
    return $this->_stockLevel;
  }
}

class Meat implements Sellable {
  private $_color;
  private $_ballsLeft;

  public function getColor() {
    return $this->_color;
  }

  public function setColor( $color ) {
    $this->_color = $color;
  }

  public function addStock( $numItems ) {
    $this->_ballsLeft += $numItems;
  }

  public function sellItem() {
    if ( $this->_ballsLeft > 0 ) {
      $this->_ballsLeft--;
      return true;
    } else {
      return false;
    }
  }

  public function getStockLevel() {
    return $this->_ballsLeft;
  }
}

class StoreManager {
  private $_productList = array();

  public function addProduct( Sellable $product ) {
    $this->_productList[] = $product;
  }

  public function stockUp() {
    foreach ( $this->_productList as $product ) {
      $product->addStock( 100 );
    }
  }
}

$tv = new Tablet;
$tv->setScreenSize( 42 );
$ball = new Meat;
$ball->setColor( "yellow" );
$manager = new StoreManager();
$manager->addProduct( $tv );
$manager->addProduct( $ball );
$manager->stockUp();
echo "<p>There are ". $tv->getStockLevel() . " " . $tv->getScreenSize();
echo "-inch Tablets and " . $ball->getStockLevel() . " " . $ball->getColor();
echo ".</p>";
$tv->sellItem();
$ball->sellItem();
$ball->sellItem();
echo $tv->getStockLevel() . " " . $tv->getScreenSize(). " ";
echo $ball->getStockLevel() . " " . $ball->getColor();
?>