如何在php中获取当天下午6点/下午7点/晚8点的unix时间戳？

How do you, in PHP, get the UNIX timestamp of every hour (on the hour [i.e. 6:00, 7:00, 8:00]) from 6am to 9pm of the present day?

for ($i=6; $i<=21; $i++)
    echo strtotime("$i:00") . '
';