5-07单利模式

<?php
//定义一个接口
interface iDbBase
{
    //数据库的操作增删查改
    static  function insert($db,$data);
    static  function select($db,$where[]);
    static  function delete($db,$where[]);
    static  function update($db, $data,$where[]);
     // return $this;//实现链式调用
   static function doConnect($dsn,$username,$password);
}
//抽象类使用php中的单利模式来连接数据库
abstract class  aDb implements  iDbBase
{
    //返回pdo的链接对象
    private function $instance;
    //private阻止此类在外部进行实例化
    private function __construct()
    {
    }
    //只能由类的自身来进行实例化
    static function doConnect($dsn,$username,$password){
        //判断$instance是否是aDb类的对象。用instanceof
        //self访问静态成员$instance
       if(!self::$instance){
           //实例化本类，传入连接参数。
           self::$instance =new PDO($dsn,$username,$password)
       }
       return self::$instance;
    }
    //self单利模式继承
      }
    //工作类
     class Db extends aDb
     {
           //数据库的操作增删查改
    static  function insert($db,$data){
 }
    static  function select($db,$where['gender'=>1,'userid'=>2]){
        //声明一个变量
        $str="";
        foreach($where as $k=>$v){
            if(count($where)>1){
                $str.=$k.'=' .$v.'and';
             }
            else{
                $str.=$k.'=' .$v;
            }
        }
     if(count($where)>1) {
         $str=substr($str,0,strlen(strlen)-4);
     }
     else{
         echo $str;
     }
        //limit限定查询表的条数
               return $db->query("SELECT * FROM 'myid' WHERE $str  LIMIT 2")->fetchAll(PDO::FETCH_ASSOC);
    }
    static  function delete($db,$where['gender'=>1,'userid'=>2]){
        return $db->query("SELECT * FROM 'myid' WHERE $str  LIMIT 1")->fetchAll(PDO::FETCH_ASSOC);
    }
   }
     //客户端代码
    $config=[
            'type'=> $type??'mysql',
            'host'=> $host='host'??'localhost',
            'dbname'=>$dbname='dbname'??'news',
            'password'=>$password='password'??'123456tyi',
            'username'=> $username='username'??'root',
        ];
       $dsn=printf('%s:host', $config['type'],$config['host'],$config['dbname']);
       $username=$config['username'];
       $password=$config['password'];
       //获取PDO连接对象
     $db=Db::doConnect($dsn,$username,$password);
     echo '<pre>';
     print_r(Db::select($db));
?>