PHP Stopwatch だが stop 機能は無い

Stopwatch.php

<?php
class Stopwatch
{
    private $_now;

    public function __construct()
    {
    }

    public function start()
    {
        $this->_now = microtime(true)*1000;
        echo("start");
    }

    public function dump()
    {
        $before = $this->_now;
        $this->_now = microtime(true)*1000;
        $diff = $this->_now - $before;
        echo($diff);
    }

}