Question

If I want to create a new object that needs certain informations like a product id or something like that but the input is bad how can I elegant manage such a case?

class Product
{
function __construct($id)
    {
    if(is_invalid_id($id))
        { return false; }
    }
}

If I initialize it this way I still get an object (since return inside a constructor doesn’t return anything). Another way would be to use exceptions which I then can catch but that’s kinda unelegant. 3rd option is to use a static function which then checks the input and then returns the object.

class Product
{
static function init($id)
    {
    if(is_invalid_id($id))
        { return false; }
    return new self($id);
    }

private function __construct($id)
    {
    $this->id = $id;
    }
}

$product = Product::init($productId);

The problem here is when I try to extend the class. Either I have to create a init() method for every class I extend (even if it is the exact same code) or return new self() always returns an instance of the parent class.

Answer 1

For the problem with self:: you might be able to use late static binding (PHP5.3+):

<?php

class Product
{
static function init($id)
    {
    if(false)
        { return false; }
    $s = get_called_class();
    return new $s($id);
    }

private function __construct($id)
    {
    $this->id = $id;
    }
function getId()
    {
    return "Product-$this->id";
    }
}


class Headphones extends Product
{
function getId()
    {
    return "Headphones-$this->id";
    }
}

$c1 = Product::init(1);
$c2 = Headphones::init(1);
printf("c1 is %s, c2 is %s\n", $c1->getId(), $c2->getId());
// Prints: c1 is Product-1, c2 is Headphones-1
?>