php计算执行时间的类

<?php
class c_Timer {
var $t_start = 0;
var $t_stop = 0;
var $t_elapsed = 0;
function start() {
$this->t_start = microtime();
}
function stop() {
$this->t_stop = microtime();
}
function elapsed() {
if ($this->t_elapsed) {
   return $this->t_elapsed;
} else {
   $start_u = substr($this->t_start,0,10); 
   $start_s = substr($this->t_start,11,10);
   $stop_u = substr($this->t_stop,0,10);  
   $stop_s = substr($this->t_stop,11,10);
   $start_total = doubleval($start_u) + $start_s;
   $stop_total = doubleval($stop_u) + $stop_s;
   $this->t_elapsed = $stop_total - $start_total;
   return $this->t_elapsed;
  }
 }
};
?>

<?php
  $timer = new c_Timer;
  $timer->start();
  echo "<hr>";
  $timer->stop();
  echo $timer->elapsed();
?>