PHP code to obtain the timestamp of a certain period of time and send letters regularly

$y=date("Y",time());
$m=date("m",time());
$d=date("d",time());
$start_time = mktime(9, 0, 0, $m, $d ,$y);
$end_time = mktime(19, 0, 0, $m, $d ,$y);
$time = time();
if($time >= $start_time && $time <= $end_time)
{
// do something....
}