Login con POO

<?php
interface Login{
	public function verificar();
}

class LoginImpl implements Login{
    private $username;
    private $password;
    private $dato = array();
	private $db;
	
	private $consulta;

    public function __construct($username,$password,$datos){
        $this->username = $username;
        $this->password = $password;
        
        $this->dato = explode(' ',$datos);
        $this->db = mysqli_connect($this->dato[0],$this->dato[1],$this->dato[2],$this->dato[3]);
	}

    public function verificar(){
        $res = false;
		$this->consulta = mysqli_query($this->db, "SELECT username,password FROM usuarios WHERE username = '".mysqli_real_escape_string($db, $this->username)."' AND password = '".mysqli_real_escape_string($db, $this->password)."' LIMIT 0,1");
        if($consulta1 = mysqli_fetch_assoc($this->consulta)){
            $res = true;
        }
        return $res;
    }
    
}

$login = new LoginImpl("usuario","password","localhost root user db");
if($login->verificar() == true){
	echo 'Correcto';
}else{
	echo 'Incorrecto';
}
$login->close();
?>