PHP5 类型提示

<?php
    /**
     * @date Tue Mar 11 01:49:44 CST 2008
     * @user BianYuan
     * @todo PHP5 类型提示
     * */
    class DB {
        function __construct() {
            // DB CONNECT

        }
    }
    class WebApp {
        private $DB;
        function __construct(DB $db) {
            $this->DB = $db;
        }
        function run() {}
    }
    $dbLink = new DB();
    $misc = null;
    $app = new WebApp($misc);

    //Catchable fatal error:  Argument 1 passed to WebApp::__construct() must be an instance of DB, null given, called in PHPDocument2 on line 21 and defined in PHPDocument2 on line 14

?>