define( 'DATA_BASE', 'db_name'   );
define( 'USER_NAME', 'user_name' );
define( 'PASSWORD' , 'password'  );
define( 'HOST'     , 'localhost' );

class MysqliDB
{
    private $user;
    private $password;
    private $database;
    private $host;

    public function __construct( $user, $password, $database, $host = 'localhost' )
    {
        $this->user     = $user;
        $this->password = $password;
        $this->database = $database;
        $this->host     = $host;
    }

   public function __construct()
    {
        $this->user     = USER_NAME;
        $this->password = PASSWORD;
        $this->database = DATA_BASE;
        $this->host     = HOST;
    }
}

class MysqliDB {

    public function __construct( $user = USER_NAME, $password = PASSWORD, $database = DATA_BASE, $host = HOST ) {}
}

new MysqliDB( null, 1234 );

public function __construct( $user = USER_NAME, $password = PASSWORD, $database = DATA_BASE, $host = HOST )
    {
        $this->user     = $user;
        $this->password = $password;
        $this->database = $database;
        $this->host     = $host;

        $this -> checkIntegrity();
    }

   public function checkIntegrity()
    {
        if( empty( $this -> username ) ) {
            die( 'Usuário MySQL ausente' );
        }
    }
}

class MysqliDB {

    public function __construct()
    {
        list( $user, $password, $database, $host ) = func_get_args();

        $this->user     = $user;
        $this->password = $password;
        $this->database = $database;
        $this->host     = $host;
    }
}

list( $user, $password, $database, $host ) = func_get_args() + array( USER_NAME, PASSWORD, DATA_BASE, HOST );

class Foo {

    function Foo() {

        // No PHP4 esse método era o construtor
    }
}

class Foo {

    function Foo() {

        echo __METHOD__, '<br />';
    }

    public function __construct() {

        echo __METHOD__, '<br />';
    }
}