php数据库操作model类(使用__call方法)

<?php
/*
作者 : shyhero
*/
define("hostname","127.0.0.1");
define("username","root");
define("password","");
define("dataname","class");
class model{
    private $link;
    private $tablename;
    private $zd;
    private $method = array(
      "where" => "",
      "order" => "",
      "limit" => "",
      "group" => "",
      "having" => ""
      );
    public function __construct($tablename){
      $this -> tablename = $tablename;
      try{
        $this -> link = mysqli_connect(hostname,username,password,dataname);
        mysqli_set_charset($this -> link,"utf8");
      }catch(exception $e){
        echo "数据库连接失败";
      }
      $this -> desc();
    }
    public function __destruct(){
      mysqli_close($this -> link);
    }
    public function desc(){
      $sql = " desc {$this -> tablename}; ";
      $res = mysqli_query($this -> link,$sql);
      $arr = mysqli_fetch_all($res,mysqli_assoc);
      for($i = 0 ;$i < count($arr);$i++){
        $brr[] = $arr[$i]['field'];
      }
      $this -> zd = $brr;
      return $brr;
    }
    public function __call($name,$value){
      $name = strtolower($name);
      if(array_key_exists($name,$this -> method)){
        if($name == 'order'){
          $this -> method['order'] = " order by ".$value[0];
        }elseif($name == 'group'){
        $this -> method['group'] = " group by ".$value[0];
        }else{
          $this -> method[$name] = " {$name} ".$value[0];
        }
      }else{
        return "the method is not found!";
      }
      return $this;
    }
    public function method(){
      return " {$this -> method['where']} {$this -> method['order']} {$this -> method['limit']} {$this -> method['group']} {$this -> method['having']}; ";
    }
    public function find($a="*"){
      if(in_array("{$a}",$this -> zd) || $a == "*"){
        $sql = "select {$a} from {$this -> tablename} {$this -> method()} ";
      }else{
        $sql = "select * from {$this -> tablename}";
      }
      //return $sql;
      $res = mysqli_query($this -> link,$sql);
      $arr = mysqli_fetch_all($res,mysqli_assoc);
      return $arr;
    }
}

<?php
  function __autoload($classname){
    require($classname.".class.php");
  }
  $a = new model("stu");
  $a -> where("name = 'zhu'")->limit("5,10");
  var_dump($a -> find("name"));