PHP计算时间段内周数

//e.g. 
//$firstDate='2017-12-5'; 
//$lastDate='2020-1-3';
function calculateWeeks($firstDate='',$lastDate='')
{
    $lastTime=date_timestamp_get(date_create($lastDate));    //1512403200
    $firstTime=date_timestamp_get(date_create($firstDate));  //1577980800

    //一年的第几周
    $lastWeekNum=date('W',$lastTime);    //01
    $firstWeekNum=date('W',$firstTime);  //49

    //周属于哪一年
    $lastWeek_Year=date('o',$lastTime);  //2020
    $firstWeek_Year=date('o',$firstTime); //2017

//中间年份周数
//2017～2020 中间有两年完整年 （2020-2017-1）*52
    $through_Years_Weeks=($lastWeek_Year-$firstWeek_Year-1)*52; //104
    $first_Year_weeks=52-$firstWeekNum+1;  //4
    $last_Year_weeks=$lastWeekNum;  //01
    $weeks=$through_Years_Weeks+$first_Year_weeks+$last_Year_weeks;    //109

    return $weeks;
 
}