Best way to do multiple constructors in PHP

I'd probably do something like this:

<?phpclass Student{    public function __construct() {        // allocate your stuff    }    public static function withID( $id ) {        $instance = new self();        $instance->loadByID( $id );        return $instance;    }    public static function withRow( array $row ) {        $instance = new self();        $instance->fill( $row );        return $instance;    }    protected function loadByID( $id ) {        // do query        $row = my_awesome_db_access_stuff( $id );        $this->fill( $row );    }    protected function fill( array $row ) {        // fill all properties from array    }}?>

Then if i want a Student where i know the ID:

$student = Student::withID( $id );

Or if i have an array of the db row:

$student = Student::withRow( $row );

Technically you're not building multiple constructors, just static helper methods, but you get to avoid a lot of spaghetti code in the constructor this way.

The solution of Kris is really nice, but I prefer a mix of factory and fluent style:

<?phpclass Student{    protected $firstName;    protected $lastName;    // etc.    /**     * Constructor     */    public function __construct() {        // allocate your stuff    }    /**     * Static constructor / factory     */    public static function create() {        return new self();    }    /**     * FirstName setter - fluent style     */    public function setFirstName($firstName) {        $this->firstName = $firstName;        return $this;    }    /**     * LastName setter - fluent style     */    public function setLastName($lastName) {        $this->lastName = $lastName;        return $this;    }}// create instance$student= Student::create()->setFirstName("John")->setLastName("Doe");// see resultvar_dump($student);?>

PHP is a dynamic language, so you can't overload methods. You have to check the types of your argument like this:

class Student {   protected $id;   protected $name;   // etc.   public function __construct($idOrRow){    if(is_int($idOrRow))    {        $this->id = $idOrRow;        // other members are still uninitialized    }    else if(is_array($idOrRow))    {       $this->id = $idOrRow->id;       $this->name = $idOrRow->name;       // etc.      }}