PHP constructor to return a NULL

Question

Assuming you're using PHP 5, you can throw an exception in the constructor:

class NotFoundException extends Exception {}

class User {
    public function __construct($id) {
        if (!$this->loadById($id)) {
             throw new NotFoundException();
        }
    }
}

$this->LoggedUser = NULL;
if ($_SESSION['verbiste_user'] != false) {
    try {
        $this->LoggedUser = new User($_SESSION['verbiste_user']);
    } catch (NotFoundException $e) {}
}

For clarity, you could wrap this in a static factory method:

class User {
    public static function load($id) {
        try {
            return new User($id);
        } catch (NotFoundException $unfe) {
            return null;
        }
    }
    // class body here...
}

$this->LoggedUser = NULL;
if ($_SESSION['verbiste_user'] != false)
    $this->LoggedUser = User::load($_SESSION['verbiste_user']);

As an aside, some versions of PHP 4 allowed you to set $this to NULL inside the constructor but I don't think was ever officially sanctioned and the 'feature' was eventually removed.

AFAIK this can't be done, new will always return an instance of the object.

What I usually do to work around this is:

• Adding a ->valid boolean flag to the object that determines whether an object was successfully loaded or not. The constructor will then set the flag

• Creating a wrapper function that executes the new command, returns the new object on success, or on failure destroys it and returns false

-

function get_car($model)
      {
        $car = new Car($model);
        if ($car->valid === true) return $car; else return false;
     } 

I'd be interested to hear about alternative approaches, but I don't know any.

Consider it this way. When you use new, you get a new object. Period. What you're doing is you have a function that searches for an existing user, and returns it when found. The best thing to express this is probably a static class function such as User::findUser(). This is also extensible to when you're deriving your classes from a base class.

A factory might be useful here:

class UserFactory
{
    static public function create( $id )
    {
        return (
            filter_var( 
                $id,
                FILTER_VALIDATE_INT, 
                [ 'options' => [ 'min_range' => 1, ] ]
            )
                ? new User( $id )
                : null
        );
  }
}