php 将某个数组里的字符串二次拆分后的某个值再次拼接成字符串

array(3) {
  [0] => array(4) {
    ["class_id"] => string(2) "16"
    ["class_type"] => string(1) "1"
    ["class_time"] => string(10) "1483545600"
    ["week_time"] => string(24) "1:12,2:12,3:12,4:12,5:12"
  }
  [1] => array(4) {
    ["class_id"] => string(2) "21"
    ["class_type"] => string(1) "1"
    ["class_time"] => string(10) "1485100800"
    ["week_time"] => string(23) "1:7,2:7,3:7,4:7,5:7,6:7"
  }
  [2] => array(4) {
    ["class_id"] => string(2) "22"
    ["class_type"] => string(1) "1"
    ["class_time"] => string(10) "1485100800"
    ["week_time"] => string(23) "1:7,2:7,3:7,4:7,5:7,6:7"
  }

}

代码:

foreach($class_info as $k=>$v){
    $vr=explode(",",$v["week_time"]);
    $str="";
    foreach($vr as $key =>$val){
        $str.=",".explode(":",$val)[0];//对吗
    }
    $class_info[$k]["weektime"]=substr($str,1);
}

array(3) {
  [0] => array(5) {
    ["class_id"] => string(2) "16"
    ["class_type"] => string(1) "1"
    ["class_time"] => string(10) "1483545600"
    ["week_time"] => string(24) "1:12,2:12,3:12,4:12,5:12"
    ["weektime"] => string(9) "1,2,3,4,5"
  }
  [1] => array(5) {
    ["class_id"] => string(2) "21"
    ["class_type"] => string(1) "1"
    ["class_time"] => string(10) "1485100800"
    ["week_time"] => string(23) "1:7,2:7,3:7,4:7,5:7,6:7"
    ["weektime"] => string(11) "1,2,3,4,5,6"
  }
  [2] => array(5) {
    ["class_id"] => string(2) "22"
    ["class_type"] => string(1) "1"
    ["class_time"] => string(10) "1485100800"
    ["week_time"] => string(23) "1:7,2:7,3:7,4:7,5:7,6:7"
    ["weektime"] => string(11) "1,2,3,4,5,6"
  }
}