How to write a function to calculate the execution time of a page in PHP

<?php
/*
@ 计算php程序运行时间
*/
function microtime_float()
{
list($usec, $sec) = explode(” “, microtime());
return ((float)$usec + (float)$sec);
}
//开始计时，放在头部
$starttime = microtime_float();
//结束计时，放在最底部
$runtime = number_format((microtime_float() – $starttime), 4).&#39;s&#39;;
//输出
echo ‘RunTime:&#39;.$runtime;
?>