PDO封裝增刪改查

<?phpclass  db{    public  $table=null;    public  $pdo;    public  $where=null;    //where 條件    public   $field=null;  //要查詢的條件  public  function  __construct()  {       $this->pdo=new PDO("mysql:host=127.0.0.1;dbname=1611b","root","root");  }    public  function  fetch(){       return  $this->pdo->query("select * from $this->table $this->where")->fetch(PDO::FETCH_ASSOC);  }    public  function   table($table){        $this->table=$table;        return $this;  }  public function  where($where){      $str="where ";      foreach ($where as $k=>$v){          $str.=$k."="."'".$v."'". " and " ;      }      $this->where=rtrim($str," and ");     return  $this;  }  public   function  insert($data){      $k=array_keys($data);      $k=implode($k,',');      $str="";      foreach ($data as $key=>$value){          $str.=","."'".$value."'";      }      $str=substr($str,1);    return     $this->pdo->exec("insert into $this->table ($k) values ($str)");  }  public   function   delect($id){      $str='';      $str1='';      foreach ($id as $k=>$v){          $str.=$k;          foreach ($v as $kk=>$vv){              $str1.=','.$vv;          }      }      $str2=substr($str1,1);      $ids='where '.$str.' in '.'('.$str2.')';      return $this->pdo->exec("delete from $this->table $ids");  }    function update($res){        //修改        $str='';        foreach ($res as $k=>$v){            $str.=','.$k.'='."'".$v."'";        }        $str=substr($str,1);        return  $this->pdo->exec("update $this->table set $str $this->where");    }  }