class Mysql{
 private $host;
 private $prot;
 private $username;
 private $password;
 private $db_name;
 // 构造方法
 public function __construct(){
   $this->host = '127.0.0.1';
   $this->port = 22;
   $this->username = 'root';
   $this->password = '';
   $this->db_name = 'my_db';
 }
 // 连接
 public function connect(){
   return mysqli_connect($this->host,$this->username,$this->password,$this->db_name,$this->port);
 }
}

$db = new Mysql();
$db->connect();

class MysqlConfiguration{
  private $host;
  private $prot;
  private $username;
  private $password;
  private $db_name;
  public function __construct($host,$port,$username,$password,$db_name){
    $this->host = $host;
    $this->port = $port;
    $this->username = $username;
    $this->password = $password;
    $this->db_name = $db_name;
  }
  public function getHost(){
    return $this->host;
  }
  public function getPort(){
    return $this->port();
  }
  public function getUsername(){
    return $this->username;
  }
  public function getPassword(){
    return $this->password;
  }
  public function getDbName(){
    return $this->db_name;
  }
}

class Mysql{
 private $configuration;
 public function __construct($config){
   $this->configuration = $config;
 }
 // 连接
 public function connect(){
   return mysqli_connect($this->configuration->getHost(),$this->configuration->getUsername(),$this->configuration->getPassword(),$this->configuration->getDbName(),$this->configuration->getPort());
 }
}

$config = new MysqlConfiguration('127.0.0.1','root','password','my_db',22);
// $config是注入Mysql的，这就是所谓的依赖注入
$db = new Mysql($config);
$db->connect();