a fixed version of this class : http://www.phpclasses.org/browse/file/33699.html

Benchmark.class.php

<?php
/**
*Benchmark class, it's a good solution for testing your web applications and scripts!
*also it can be a permanent tool you can integrate into your systems that will indicate the performance of your app/script etc.
* Test Time and memory consumption!
* Example usage:
*
* <code>
* require('Benchmark.class.php');
* Benchmark::startTimer();
* echo Benchmark::showTimer(5) . ' sec. ';
* echo Benchmark::showMemory('kb') . ' kb' ;
* </code>
*
*@author David Constantine Kurushin
*@link http://www.zend.com/en/store/education/certification/authenticate.php/ClientCandidateID/ZEND015209/RegistrationID/238001337
*@version 10.12.2010
*@package Benchmark
*@copyright Copyright (c) 2010, David Constantine Kurushin
 */
class Benchmark{

    /**
    *Class Fields startTime and stopTime
    * @var double $sartTime holds the starting time
    * @var double $stopTime holds the ending time
     */
    private $startTime , $stopTime; //no static. also no need to set null
    
    /**
    * reformats a microtime string
    * @param string a microtime string
    *@return full microtime 
     */
    private function getm($time){
        return array_sum(explode(" ",$time);
    }

    /**
    * starts the timer
     */
    public function startTimer(){//Benchmark::startTimer();
        $this->$startTime = microtime();
        }

    /**
    * stops the timer
     */
    private function stopTimer(){//Benchmark::stopTimer();
            $this->$stopTime=microtime();
    }

    /**
    *Calculate the time elapsed from starting to stopping in second
    *@param integer $round set how much numbers you want after Zero, default is 10
    *@return double the time elapsed from starting to stopping in seconds
     */
    public function showTimer($round=10){
        if(!$this->$startTime)
            throw new LogicException('A timer must be started before a time can be measured');
        else 
            $this->$stopTime=$this->getm();
        return number_format( ($this->getm($this->$stopTime) - $this->getm($this->$startTime)) , $round, '.', '');
    }
    
    /**
    *Calculate the memory that used/alocated to your script
    *@param String $string you should choose the format you want, 'mb'/'kb'/'bytes' default if bytes!
    *@param integer $round set how much numbers you want after Zero, default is 3
    *@return double amount of memory your script consume
     */
    public function showMemory($string ='bytes', $round=3){
        $result=null;
        switch($string){
            case 'mb': $result = round(memory_get_usage()/1048576,$round);
            break;
            case 'kb': $result = round(memory_get_usage()/1024, $round);
            break;
            default: $result = memory_get_usage();
            break;
        }
        return $result;
    }
    
    public function __toString(){
        return $this->showTimer();
    }
}

usage example:
startTimer(); echo $bm->showTimer(); echo $bm->showTimer(); echo (string)$bm; ?>
notice also that until you recall startTimer, you can get multiple endtimes compared to it