如何解决oop错误

Question

I'm newbie here. I tried to solve this for a day and searching all the way but I still couldn't.

The error shown up

Notice: Undefined variable: db (in dbFunction)

Fatal error: Call to a member function query() on a non-object (in dbFunction)

and code is

dbConnect.php

class dbConnect{

    public $db;

    public function __construct(){
        $db = new mysqli("localhost", "root", "", "onlineshopping");
        $db->set_charset("charset");
        if (mysqli_connect_errno()) {
            printf("Connect failed: %s
", mysqli_connect_error());
            exit();
        }
    }   
}

dbFunction

class dbFunction extends dbConnect {  

    function __construct() {  

    }  

    public $db;

    public function UserRegister($fname, $lname, $tel, $email, $pass){
        $pass = md5($pass);  
        $db->query("INSERT INTO member(fname, lname, tel, email, password) values('".$fname."','".$lname."','".$tel."','".$email."','".$pass."')") or die(mysqli_error());  
        return $db;     
    }  
}

Answer 1

First of all, your $db is a local variable in the __construct() method and not a class property and second, as pointed by @wogsland it gets overwritten by the child.
You might want to review a bit the OO basics

dbConnect.php

class dbConnect{

    public $db;

    public function __construct(){
        $this->db = new mysqli("localhost", "root", "", "onlineshopping");
        $this->db->set_charset("charset");
        if (mysqli_connect_errno()) {
            printf("Connect failed: %s
", mysqli_connect_error());
            exit();
        }
    }   
}

dbFunction

class dbFunction extends dbConnect {  

    public function UserRegister($fname, $lname, $tel, $email, $pass){
        $pass = md5($pass);  
        $this->db->query("INSERT INTO member(fname, lname, tel, email, password) values('".$fname."','".$lname."','".$tel."','".$email."','".$pass."')") or die(mysqli_error());       
    }  
}

Notice the replacement of $db with $this->db in both methods.

edit:

My post was an exact answer to your question but, as others have pointed in comments, there are quite a few things you need to improve and my hope was that this code is for your own personal play not something for 'production'.

So, directly related to the code example, two obvious things you might want to expand:

• at least escape all your input to avoid sql injection
• don't use MD5 for hashing passwords as it is very weak, check a more secure way to hash passwords