PHP OOP连接到MYSQL(构造函数与传递实例）的效率

我一直在想OOP连接的效率。示例结构如下所示。每个类由文件夹和类名分隔。例如：

/mysql/mysql.class.php //holds general connect and disconnect functions
/mysql/query.class.php //holds all the queries made to the db (uses basic mysql ^ )
/library/application.class.php //holds all the algorithms (uses queries to db ^ )

当涉及到执行算法时，application.class.php调用query.class.php（当它需要查询时），后者调用mysql.class.php进行连接。

这两种方法中哪一种是正确的（如果有的话，或者两者都是）？有没有一种更简单、更有效的方法可以做到这一点？

这是做两件事的相同方法（哪个更有效？）：

require_once('/mysql/mysql.class.php ');
require_once('/mysql/query.class.php ');
$mysqlClass = new MYSQL();
$queryClass = new Query();
class Application {                
  public function getVar() {         
    $param = foo;         
    $db = $mysqlClass->connect(); //connection to mysql
    $queryClass->callsomequerymethod($db, $param); //pass the db connection to function
  } 
} 

VS

class Application {     
  private $db;      
  public function __construct(DatabaseConnection $db, QueryClass $queryclass){
    $this->db = $db;     
  }           
  public function getVar() {         
    $param = foo;         
    $db = $this->db;
    $queryclass->callsomequerymethod($db, $param);
  } 
}