PHP microtime用法点滴

<?php
class runTime {
    var $StartTime = 0;
    var $StopTime = 0;
    var $TimeSpent = 0;

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

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

    function spent() {
        if ($this->TimeSpent) {
            return $this->TimeSpent;
        } else {
            $StartMicro = substr($this->StartTime,0,10);
            $StartSecond = substr($this->StartTime,11,10);
            $StopMicro = substr($this->StopTime,0,10);
            $StopSecond = substr($this->StopTime,11,10);
            $start = floatval($StartMicro) + $StartSecond;
            $stop = floatval($StopMicro) + $StopSecond;
            $this->TimeSpent = $stop - $start;
            return round($this->TimeSpent,8);
        }
    } // end function
}

<?php
function microtime_float3(){
    return microtime(true);
}

function microtime_float2(){
    if( PHP_VERSION > 5){
        return microtime(true);
    }else{
        list($usec, $sec) = explode(" ", microtime());
        return ((float)$usec + (float)$sec);
    }
}

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

function runtime($t1){
    return number_format((microtime_float() - $t1)*1000, 4).'ms';
}

$t1 = microtime_float();
for($i=0;$i<10000;$i++){
    microtime_float();
}
echo "microtime_float=====";
echo runtime($t1).'<br>';
$t1 = microtime(true);

for($i=0;$i<10000;$i++){
    microtime(true);
}
echo "microtime_true=====";
echo runtime($t1).'<br>';
$t1 = microtime(true);

for($i=0;$i<10000;$i++){
    microtime_float2();
}

echo "microtime_float2=====";
echo runtime($t1).'<br>';
$t1 = microtime(true);

for($i=0;$i<10000;$i++){
    microtime_float3();
}
echo "microtime_float3=====";
echo runtime($t1).'<br>';
?>