A simple timer class

Timer.php

<?php
/* The MIT License (MIT)
 *
 * Copyright (c) 2015 Stephen Coakley
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/**
 * A class for measuring precise timings.
 */
class Timer
{
    /**
     * @var int The timestamp the timer started at.
     */
    protected $startTime = 0;

    /**
     * @var int[] List of lap times.
     */
    protected $laps = array();

    /**
     * @var bool Whether the timer is currently running.
     */
    protected $running = false;

    /**
     * Checks if the timer is running.
     *
     * @return bool True if the timer is running, otherwise false.
     */
    public function isRunning()
    {
        return $this->running;
    }

    /**
     * Starts the timer.
     */
    public function start()
    {
        $this->running = true;

        if ($this->startTime === 0) {
            $this->startTime = microtime(true) * 1000;
        }
    }

    /**
     * Stops the timer if it is currently running.
     *
     * @return int The time the timer stopped at.
     */
    public function stop()
    {
        $this->running = false;
        return $this->getTimeElapsed();
    }

    /**
     * Adds a lap marker.
     *
     * @return int The time of the lap.
     */
    public function lap()
    {
        if ($this->running) {
            $time = $this->getTimeElapsed();
            array_push($this->laps, $time);
            return $time;
        }
    }

    /**
     * Resets the timer back to zero.
     */
    public function reset()
    {
        $this->startTime = 0;
        $this->laps = array();
    }

    /**
     * Gets the amount of time elapsed in milliseconds.
     *
     * @return int The number of milliseconds past since the timer began.
     */
    public function getTimeElapsed()
    {
        return (microtime(true) * 1000) - $this->startTime;
    }

    /**
     * Gets the lap times in milliseconds.
     *
     * @return int[] A list of lap times since the timer started.
     */
    public function getLaps()
    {
        return $this->laps;
    }
}