Tracking time elapsed for php program in millisecond

runtime.php

<?php

# Tracking time elapsed for php program in millisecond
# by @leaskh

class runtime {
    var $StartTime = 0;
    var $StopTime = 0;

    function get_microtime() {
        list($usec, $sec) = explode(' ', microtime());
        return ((float)$usec + (float)$sec);
    }

    function start() {
        $this->StartTime = $this->get_microtime();
    }

    function stop() {
        $this->StopTime = $this->get_microtime();
    }

    function spent() {
        return round(($this->StopTime - $this->StartTime) * 1000, 1);
    }

}

$aa=new runtime();
$aa->start();
echo file_get_contents("1.txt")."<br>";
$aa->stop();
echo $aa->spent();