PHP constructor to return a NULL

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.