PHP class constructor in interface or class

Question

I'm having some issues thinking out a good structure to build my classes and objects. In the code below I make use of an interface to define my class methods, but I also want to pass a database connection to my constructor so the class had this connection to work with. Is it correct as I coded it below that the constructor is placed within my class and the methods in my interface?

interface IDataItem
{
    public function saveItem(Item $theItem);
}

class DataItem implements IDataItem
{
    public function __construct(Database $database) 
    { 
        $this->database = $database;
    }

    public function saveItem(Item $item) 
    {       
        //save the item
    }
}

$db = new Database(); //from a database class
$dataItem = new DataItem($db);          
$dataItem->saveItem($anItem);

Answer 1

While it is technically possible to add the constructor to the Interface, Interfaces should not define the constructor because that would be an implementation detail of an implementing class. An Interface should just define the public API other collaborators can call upon. That is, they should not enforce a particular implementation.

If you'd put a constructor asking for a database connection in the Interface, you'd limit the concrete classes to the dependencies in the constructor signature. If a concrete class implementing the Interface needs different (because it's saving to a Webservice) or additional (maybe a Logger) dependency you cannot make that work with your Interface.

Answer 2

I don't personally think you should put the constructor in the interface because you'd never create a new object by calling the constructor without being aware of which implementation you're using.

There is a mistake in your code, the method in an interface cannot have an implementation, it needs to be just a declaration, like this

interface IDataItem
{
    public function saveItem($theItem);
}