php单例模式封装数据库操作类增删改查

<?php
//三私一公 单例
class Db{
   //数据库连接对象
   private static $instance;
   private static $table_name;
   private $pdo;
   //防止类直接实例化
   private function __construct(){
      $this->pdo = new PDO("mysql:host=127.0.0.1;dbname=test_system", "root", "");
      $this->pdo->query("set names utf8");
   }
   //禁止克隆对象
   private function __clone(){}
   //返回数据库实例对象
   public static function getDb($table_name){
      self::$table_name = $table_name;
      if(!(self::$instance instanceof self)){
         self::$instance = new self;
      }
      return self::$instance;
   }
   function add($table_name, $data){
      $keys = implode(",", array_keys($data));
      $value = "'".implode("','", array_values($data))."'";
      $sql = "insert into $table_name ($keys) values($value) ";
      $r = $this->pdo->exec($sql);
      $this->getErrorInfo();
      return $r;
   }
   function addAll($table_name, $data){
      $keys = implode(",", array_keys($data[0]));
      $arr = [];
      foreach ($data as $k => $v) {
         $arr[] = "('".implode("','", array_values($v))."')";
      }
      $value = implode(",", $arr);
      $sql = "insert into $table_name ($keys) values $value";
      $r = $this->pdo->exec($sql);
      $this->getErrorInfo();
      return $r;
   }
   function update($table_name, $data){
      $id = $data['id'];
      unset($data['id']);
      $arr = [];
      foreach($data as $k=>$v){
         $arr[] = $k."='".$v ."'";
      }
      $str = implode(",", $arr);
      $sql = "update $table_name set $str where ;
      $r = $this->pdo->exec($sql);
      $this->getErrorInfo();
      return $r;
   }
   function select($table_name, $where = '1=1'){
      $sql = "select * from $table_name where $where ";
      $res = $this->pdo->query($sql)->fetchAll(PDO::FETCH_ASSOC);
      $this->getErrorInfo();
      return $res;
   }
   function find($table_name, $where = '1=1'){
      $sql = "select * from $table_name where $where ";
      $res = $this->pdo->query($sql)->fetch(PDO::FETCH_ASSOC);
      $this->getErrorInfo();
      return $res;
   }
   function getErrorInfo(){
      if($this->pdo->errorCode() != '00000'){
         echo "<pre>";
         print_r($this->pdo->errorInfo());
         exit;
      }
   }
   function delete($id){
      $table_name = self::$table_name;
      if(is_array($id)){
         $id = implode(',', $id);
      }
      $sql = "delete from $table_name where id in ($id)";
      $r = $this->pdo->exec($sql);
      $this->getErrorInfo();
      return $r;
   }
}
function M($table_name){
   $db = Db::getDb($table_name);
   return $db;
};
$data = [
   [
      'name'=>'雪碧',
      'class_name'=>'3333333',
   ],
   [
      'name'=>'可乐',
      'class_name'=>'3333333',
   ],
];
$r = M('user')->delete(726);
echo $r;