<?php
//string '2017-02-26 10:40:12' (length=19)
$res= array(
      'id' =>  '172',
      'user_id' => '13',
      'type' =>  '签到' ,
      'typename' => 'checkin' ,
      'id_status' => 'up',
      'points' =>  '5',
      'remark' =>  '奖励5积分',
      'createtime' => '2017-02-27 13:40:42'
      )
      $sign='[';
        foreach($res as $key=>$value){
            $first=explode(' ', $value['createtime']);
            $second=explode('-', $first['0'])['2'];
            if($key==0){
                $sign .= '{"signDay":"'.$second.'"}';
            }else{
                $sign .= ',{"signDay":"'.$second.'"}';
            }
        }
        $sign .=']';
    echo json_encode($sign);
?>