原理：简单的把开始时间放在页面头部，结束时间放在页面尾部，计算页面加载时间。

$start = time();
// put a long operation in here
sleep(2);
$diff = time() - $start;
print "This page needed $diff seconds to load :-)";
// if you want a more exact value, you could use the microtime function