Explain the relevant content of php to obtain the specified date

<?php/**
 * 计算指定日期的一周开始及结束日期
 * @param  DateTime $date  日期
 * @param  Int      $start 周几作为一周的开始 1-6为周一~周六，0为周日，默认0
 * @retrun Array
 */function getWeekRange($date, $start=0){    // 将日期转时间戳
    $dt = new DateTime($date);    $timestamp = $dt->format(&#39;U&#39;);    // 获取日期是周几
    $day = (new DateTime(&#39;@&#39;.$timestamp))->format(&#39;w&#39;);    // 计算开始日期
    if($day>=$start){        $startdate_timestamp = mktime(0,0,0,date(&#39;m&#39;,$timestamp),date(&#39;d&#39;,$timestamp)-($day-$start),date(&#39;Y&#39;,$timestamp));
    }elseif($day<$start){        $startdate_timestamp = mktime(0,0,0,date(&#39;m&#39;,$timestamp),date(&#39;d&#39;,$timestamp)-7+$start-$day,date(&#39;Y&#39;,$timestamp));
    }    // 结束日期=开始日期+6
    $enddate_timestamp = mktime(0,0,0,date(&#39;m&#39;,$startdate_timestamp),date(&#39;d&#39;,$startdate_timestamp)+6,date(&#39;Y&#39;,$startdate_timestamp));    $startdate = (new DateTime(&#39;@&#39;.$startdate_timestamp))->format(&#39;Y-m-d&#39;);    $enddate = (new DateTime(&#39;@&#39;.$enddate_timestamp))->format(&#39;Y-m-d&#39;);    return array($startdate, $enddate);
}?>

<?php$date = &#39;2016-04-27&#39;;for($start=0; $start<=6; $start++){    list($startdate, $enddate) = getWeekRange($date, $start);    echo &#39;date:&#39;.$date.&#39; week start:&#39;.$start.&#39; range:&#39;.$startdate.&#39;, &#39;.$enddate.&#39;<br>&#39;;
}?>

date:2016-04-27 week start:0 range:2016-04-24, 2016-04-30date:2016-04-27 week start:1 range:2016-04-25, 2016-05-01date:2016-04-27 week start:2 range:2016-04-26, 2016-05-02date:2016-04-27 week start:3 range:2016-04-27, 2016-05-03date:2016-04-27 week start:4 range:2016-04-21, 2016-04-27date:2016-04-27 week start:5 range:2016-04-22, 2016-04-28date:2016-04-27 week start:6 range:2016-04-23, 2016-04-29