According to the given date format 'YYYY-mm-dd', get the starting date of the week where the date is located

Common date operations

/**
 * Get the start date of all weeks in the year
 * @param $year format 'YYYY'
 * Return the two-dimensional array subscript key1 value corresponding to the actual year of the week in which a certain day of the year is located, and the key2 value corresponding to the certain day of the year The day of the week [week of the year]
 * Returns false The date format is wrong
*/
function get_all_weeks($year){
 $week_arr = array();
 $year = intval(trim($year));
 $preg = "/^d{ 4,}$/";
 if(!preg_match($preg, $year)){
 return false;
 }
 $begin_day = $year . '-01-01';
 $end_day = $year . '- 12-31';
 //The first Monday of the year
 if(date('w',strtotime($begin_day))!=1){
 $begin_day = date('Y-m-d',strtotime(" next monday", strtotime($begin_day)));
 }
 //The number of weeks where the first Monday is located
 $begin_week_num = intval(date('W',strtotime($begin_day)));
 //One year Last Sunday
 if(date('w',strtotime($end_day))!=0){
 $end_day = date('Y-m-d',strtotime("last sunday",strtotime($end_day)));
 //There is a New Year's Eve week, and the New Year's Eve week is Monday
 $end_day_next = date('Y-m-d',strtotime($end_day)+24*60*60);
 //The year and location of the New Year's Eve week Week number
 $stride_year = date('o',strtotime($end_day_next));
 $stride_weeknum = intval(date('W',strtotime($end_day_next)));
 }
 //The number of weeks in which the last Sunday is located
 $end_week_num = intval(date('W',strtotime($end_day)));
 //The first Monday of the year is the first or second week of the year
 if($begin_week_num!=1){
 $i=2;
 }else{
 $i=1;
 }
 $j = 0;
 for($i;$i<=$end_week_num;$i++){
 $start_date = date("Y-m-d" , strtotime("$begin_day $j week"));
 $end_day = date("Y-m-d", strtotime($start_date . '+6 day'));
 $week_arr[$year][$i] = array(
 $start_date,
 $end_day
 );
 $j++;
 }
 if($end_day_next){
 $week_arr[$stride_year][$stride_weeknum] = array(
 $end_day_next,
 date("Y-m-d", strtotime( $end_day_next . '+6 day'))
 );
 }
 return $week_arr;
}
//demo_call
/*
$year = '2013';
if(get_all_weeks($year)){
 var_dump(get_all_weeks($year));
}else{
 echo 'Date format error';
}
*/
/**
 * Get the start date of the week where a certain day is located
 * Depends on the function get_all_weeks
 * @param $day format: 'YYYY-mm-dd'
 * Returns false because the date format is wrong
 * Correct, returns json "{"begin_day": "YYYY-mm-dd","end_day":"YYYY-mm-dd"}"
*/
function get_day_week($day){
 $date_arr = explode(' -', trim($day));
 if(!checkdate(intval($date_arr[1]), intval($date_arr[2]), intval($date_arr[0]))){
 return false;
 }
 $year = date('Y',strtotime($day));
 $weeks = get_all_weeks($year);
 //The year and week number of a certain day
 $real_year = date('o',strtotime ($day));
 $week_num = intval(date('W',strtotime($day)));
 if(!empty($weeks[$real_year][$week_num][0]))
 $begin_day = $weeks[$real_year][$week_num][0];
 if(!empty($weeks[$real_year][$week_num][1]))
 $end_day = $weeks[$real_year][$week_num] [1];
 //New year week value is stored in the previous year
 if(empty($begin_day)||empty($end_day)){
 $year = date('Y',strtotime($day)) -1;
 $weeks = get_all_weeks($year);
 $real_year = date('o',strtotime($day));
 $week_num = intval(date('W',strtotime($day)));
 if(!empty($weeks[$real_year][$week_num][0]))
 $begin_day = $weeks[$real_year][$week_num][0];
 if(!empty($weeks[$real_year ][$week_num][1]))
 $end_day = $weeks[$real_year][$week_num][1];
 }
 $the_day = array(
 'begin_day' => $begin_day,
 'end_day' => $end_day
 );
 $the_day = json_encode($the_day);
 return $the_day;
}
//demo_call
/*
$day = '2014-01-01';
if(get_day_week ($day)){
 var_dump(get_day_week($day));
}else{
 echo 'Date format error';
}
*/