php 删掉空的数组 json数据. 空数据（false 0 ““ null）

<?php
//删除数组中代表空的数组.
/*$entry =[
             0 => 'foo',   
             1 => false,   
             2 => -1,   
             3 => null,   
             4 => '' 
          ];
      foreach($entry as $k=>$val)
      {
        if(!$val)
        {
          unset($entry[$k]);
        }
} 
      print_r($entry);*/
echo "以下是json数据的操作";
 //删除json中空的json
    $json_string= '[{"id":"1","name":"-1","age":"27","subject":false},{"id":"2","name":"liwen","age":null,"subject":""}]';
 $data = json_decode($json_string,true);//把json转换成数组
 foreach($data as $k=>$val)//遍历数组
 {
  // print_r($val);exit();
   foreach($val as $kk=>$v)//拿到每一个$val转化成的数组
   {
      if(!$data[$k][$kk])//获取每一个元素，如果是空，就删掉
      {
        unset($data[$k][$kk]);
      }
   }
  //$data['0']["name"]="aas";
   //$data[$k][$val]="666";
   $json_strings = json_encode($data);//在转化成json，
 }
var_dump($json_strings);