在接口被实例化以后的每个方法中return 类本身就可以达到链式操作。
改善了写多行代码执行一次操作的缺点。
下面请看我的代码，若有疑问，请留言给我。

class Instance{
 
    public static $db;
 
    public $mysql=NULL;
 
    private function Connect($host,$user,$pwd,$port=3306){
        $this->mysql=mysqli_connect($host,$user,$pwd,$port) or die('Can not Connect Mysql!');
    }
 
    private function __construct(){}
 
    private function __clone(){}
 
    public  static function getInstance($host,$user,$pwd,$port=3306){
        if(!isset(self::$db)){
            self::$db = new self;
            self::$db->Connect($host,$user,$pwd,$port);
        }
        return self::$db;
    }
 
}
 
 
interface SQL{
    /**
     * @param string $sql SQL语句
     * @return mixed
     */
    public function Query($sql);
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Select($condition=array());
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Where($condition=array());
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Group($condition=array());
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Limit($condition=array());
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Order($condition=array());
 
    /**
     * @return mixed 关闭数据库连接
     */
    public function Close();
 
}
 
 
 
class CoherentModel implements SQL{
 
    private   $db; //数据库资源
 
    protected $sql; //SQL语句
 
    protected $result; //结果
 
    public function __construct($db){
        if(empty($db->mysql)){
            exit('db not connected !');
        }
        $this->db=$db->mysql;
    }
 
    public function Query($sql=''){
        $this->sql=$sql;
        $this->result=mysqli_query($this->db,$sql);
        return $this->result;
    }
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Select($condition=array()){
 
        return $this;
    }
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Where($condition=array()){
 
        return $this;
    }
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Group($condition=array()){
 
        return $this;
    }
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Limit($condition=array()){
 
        return $this;
 
    }
 
    /**
     * @param array $condition 条件
     * @return mixed
     */
    public function Order($condition=array()){
 
        return $this;
 
    }
 
    /**
     * @return mixed 关闭数据库连接
     */
    public function Close(){
        mysqli_close($this->db);
    }
}
 
/**
 * 单例模式
 */
$Instance = Instance::getInstance('127.0.0.1','mysql','password');
 
$Model = new CoherentModel($Instance::$db);
 
/**
 * 链式操作
 */
$Model->Where()->Group()->limit()->Select();