A php class that calculates program running time

/**
* Calculate program running time
* filename: js_runtime.php
*/
class Timer {
private $StartTime = 0;//Program running start time
private $StopTime = 0;//Program running end time
private $TimeSpent = 0;//It takes time for the program to run
function start(){//The program starts to run
$this->StartTime = microtime();
}
function stop(){//The program ends to run
$this ->StopTime = microtime();
}
function spent(){//The time it takes for the program to run
if ($this->TimeSpent) {
return $this->TimeSpent;
} else {
list ($StartMicro, $StartSecond) = explode(" ", $this->StartTime);
list($StopMicro, $StopSecond) = explode(" ", $this->StopTime);
$start = doubleval( $StartMicro) + $StartSecond;
$stop = doubleval($StopMicro) + $StopSecond;
$this->TimeSpent = $stop - $start;
return substr($this->TimeSpent,0,8). "Seconds";//Return the obtained program running time difference
}
}
}
$timer = new Timer();
$timer->start();
//...The program running code
$ timer->stop();
echo "The program running time is:".$timer->spent();
?>