怎么计算一个程序的执行时间

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

$time_start = getmicrotime();//开始计时, 放在程序头

for ($i=0; $i < 1000; $i++){
//do nothing, 1000 times
}

$time_end = getmicrotime();//结束计时, 放在尾部
$time = $time_end - $time_start;

echo "Did nothing in $time seconds";