Fatal error: Declaration of .. must be compatible

Question

I'm getting the following error:

Fatal error: Declaration of Shoppingcart::addToCart() must be compatible with that of Ishoppingcart::addToCart() in klassen.php on line 118

What could be the problem? I can't find it script:

<?php
// begin
interface Ishoppingcart{
    public function addToCart();
    public function printOverzicht();
}
abstract class Shoppingcart implements Ishoppingcart
{
    protected $producten = array();

    public function addToCart( Product $product) {
        $this->producten[] = $product;
    }
}
class Myshoppingcart extends Shoppingcart {
    public function printOverzicht(){
        echo ("<table border=1>
        <tr>
        <td colspan='7'>Bestellingoverzicht</td>
        </tr>
        <tr>
        <td bgcolor=#cccccc> Product ID</td>
        <td bgcolor=#cccccc> Beschrijving</td>
        <td bgcolor=#cccccc> Merk</td>
        <td bgcolor=#cccccc> Model</td>
        <td bgcolor=#cccccc> Prijs</td>
        <td bgcolor=#cccccc> Aantal</td>
        <td bgcolor=#cccccc> Korting</td>
        </tr>");

        foreach($this->producten as $product){
            $rij = "";
            $rij .= "<tr>";
            $rij .= "<td>".$product->getID()."</td>";
            $rij .= "<td>".$product->getBeschrijving()."</td>";
            $rij .= "<td>".$product->getMerk()."</td>";
            $rij .= "<td>".$product->getModel()."</td>";
            $rij .= "<td>".$product->getPrijs()."</td>";
            $rij .= "<td>".$product->getAantal()."</td>";
            $rij .= "<td>".$product->getKorting()."</td>";
            $rij .= "</tr>";
            echo ($rij);
        }
        echo ("</table>");
    }
}
class Product {
    private $id;
    private $beschrijving;
    private $merk;
    private $model;
    private $prijs;
    private $aantal;
    private $korting;

    function __construct($id,
                        $merk,
                        $model,
                        $prijs,
                        $aantal,
                        $korting){
        $this->id = $id;
        $this->beschrijving = $beschrijving;
        $this->merk = $merk;
        $this->model = $model;
        $this->prijs = $prijs;
        $this->aantal = $aantal;
        $this->korting = $korting;
        echo ("<br />Nieuw Product object $beschrijving wordt aangemaakt");
                        }
    public function __destruct(){
        // voer benodigde acties uit
        echo ("<br />Product object $this->beschrijving wordt verwijderd");
    }
    // set function
    public function setId($id){
        $this->id = $id;
    }
    public function setBeschrijving($beschrijving){
        $this->beschrijving = $beschrijving;
    }
    public function setMerk($merk){
        $this->merk = $merk;
    }
    public function setModel($model){
        $this->model = $model;
    }
    public function setPrijs($prijs){
        $this->prijs = $prijs;
    }
    public function setAantal($aantal){
        $this->aantal = $aantal;
    }
    public function setKorting($korting){
        $this->korting = $korting;
    }
    // get function
    public function getId(){
        return $this->id = $id;
    }
    public function getBeschrijving(){
        return $this->beschrijving;
    }
    public function getMerk(){
        return $this->merk;
    }
    public function getModel(){
        return $this->model;
    }
    public function getPrijs(){
        return $this->prijs;
    }
    public function getAantal(){
        return $this->aantal;
    }
    public function getKorting(){
        return $this->korting;
    }

    // printProductInfo
    public function printProductInfo(){
    $rij = "<tr><td>$this->id</td>";
    $rij .= "<td>$this->beschrijving</td>";
    $rij .= "<td>$this->merk</td>";
    $rij .= "<td>$this->model</td>";
    $rij .= "<td>$this->prijs</td>";
    $rij .= "<td>$this->aantal</td>";
    $rij .= "<td>$this->korting</td>";  
    echo ($rij);
    }
}
// einde
?>

Answer 1

The declaration of a public function in a sub class must match that of its parent:

public function addToCart();
public function addToCart( Product $product)

You cannot add a parameter to the signature.

This is closely related to the Liskov substitution principle.

Answer 2

Ishoppingcart::addToCart() states that the method does not take any parameter, while the implementation Shoppingcart::addToCart(Product $product) requires that a parameter of type Product must be passed into the method. This means that both declarations are incompatible and while the implemented interface must be satisfied PHP throws the shown error.

Solution would be to either change Ishoppingcart::addToCart() to Ishoppingcart::addToCart(Product $product) so that it requires a parameter of type Product or to change Shoppingcart::addToCart(Product $product) to allow no parameter to passed into the method: Shoppingcart::addToCart(Product $product = null);

The correct way depends on your application requirements.

Answer 3

Interface Ishoppingcart seems to define addToShoppingCart without parameters, but class Shoppingcart defines the same function taking Product as a parameter. I suppose the method in the interface should take Product as a parameter as well.