สอบคำแนะนำครับ " ผมเขียน OOP ถูกวิธีหรือเปล่า ? " กำลังเริ่มเขียนแบบ OOP น่ะครับ

<?php 
Session_Start();

include("function_connection.php");

class _system extends _connection {
	
	private $username;
	private $oldpass;
	private $newpass;
	
	public function data($result) {
		$this->username=$_SESSION['username'];
		$strSQL = mysql_query("SELECT * FROM login WHERE userid = $this->username");
		$Result = mysql_fetch_array($strSQL);
		return $Result[$result];
	}
	
	public function changepass() {
		$this->host();
		$this->db();
		$this->username=$_SESSION['username'];
		$this->oldpass=$_POST['oldpass'];
		$this->newpass=$_POST['newpass'];
		if($this->data('user_pass') != $this->oldpass) { die("Old password is correct."); } else {
		$strSQL = mysql_query("UPDATE login SET user_pass = $this->newpass WHERE userid = $this->username") or die(mysql_error());
		echo "<script>window.location='../member.php';</script>";
		}
	}
	
}

	$obj = new _system;
	$obj->changepass();

?>