PHP test program run time class

Class is very simple, mainly using several functions array list function list(), string split into array function explode(), get the timestamp and microseconds microtime(), the code is as follows:

 
<?php 
class runTime { 
private $starTime;// The start time  
private $stopTime;// The end of time  
private function getMicTime(){ 
$mictime=microtime();// Gets the timestamp and the number of microseconds  
list($usec,$sec)=explode(" ",$mictime);// Divide the microseconds into arrays and convert them into variables  
return (float)$usec+(float)$sec;// The converted data is forced to be processed by floating point  
} 
public function star(){// Get start time  
$this->starTime=$this->getMicTime(); 
} 
public function stop(){// Get end time  
$this->stopTime=$this->getMicTime(); 
} 
public function spent(){// Calculate the duration of the program  
return round($this->stopTime-$this->starTime)*1000;// Get the number of milliseconds  
} 
} 
// For example,  
$time=new runTime(); 
$time->star(); 
for ($i=1;$i<=1000;$i++){ 
echo("a"); 
} 
$time->stop(); 
echo $time->spent(); 
?>