PDO封装增删改查

<?php
class  db{

    public  $table=null;
    public  $pdo;
    public  $where=null;    //where 条件
    public   $field=null;  //要查询的条件

  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");
    }

}