Fatal error: Uncaught Error: Call to a member func

问题:

I'm a newbie to OOP and still experimenting, can someone point out where my error is? I'm getting error

Fatal error: Uncaught Error: Call to a member function query() on null on the following line:

I have realized that the DB connection is not being inherited from the Core class, but I have no idea why.

$user = $this->db->query("SELECT * FROM users WHERE id='".$user_id."' LIMIT 1");

class.php

class Core {
        public $db;
        public $domain;
        public $settings;
        public function getDomain() {
            return $this->domain;
        }
        function __construct($db,$domain) {
            $this->db = $db;
            $this->domain = $domain;
            $this->getSettings();
        }
        public function getSettings() {
            $settings = $this->db->query("SELECT * FROM settings");
            $settings = $settings->fetch_object();
            $this->settings = $settings;
        }
    }

    class User extends Core {
        public $user;
        function __construct($user_id) {
            $this->setUser($user_id);
        }
        public function setUser($user_id) {
            $user = $this->db->query("SELECT * FROM users WHERE id='".$user_id."' LIMIT 1");
            if($user->num_rows >= 1) {
                $user = $user->fetch_object();
                $this->user = $user;
            }
        }

index.php

$core = new Core($db,$domain);
$user = new User($_SESSION['user_id']);

Clarification:

My db variable is passed from my config.php file which I include at the beginning of the index.php page config.php

$_db['host'] = 'localhost';
$_db['user'] = 'root';
$_db['pass'] = 'root';
$_db['name'] = 'social';
$db = new mysqli($_db['host'], $_db['user'], $_db['pass'], $_db['name']) or die('MySQL Error');

回答1:

The problem right now lies within the way your calling the classes. The Core class __construct() function expects the $db connection. Right now, the User class will have to call the constructor function of the Core class to initiate the db connection. So you have one option at least:

Call only the User classes like this:

$user = new User($db,$domain,$_SESSION['user_id']);

while changing your User constructor to do this:

function __construct($db,$domain,$user_id) {
    parent::__construct($db,$domain);
    $this->setUser($user_id);
}

If your database connection variables are all correct (host/username/password/port), it should work.

回答2:

If there is a method of constructing a subclass, the subclass of the parent class will not be used if the method is called directly

class Core {
    public $db;
    public $domain;
    public $settings;
    public function getDomain() {
        return $this->domain;
    }
    function __construct($db,$domain) {
        $this->db = $db;
        $this->domain = $domain;
        $this->getSettings();
    }
    public function getSettings() {
        $settings = $this->db->query("SELECT * FROM settings");
        $settings = $settings->fetch_object();
        $this->settings = $settings;
    }
}

class User extends Core {
    public $user;
    function __construct($user_id,$db,$domain) {
        parent::__construct($db,$domain);
        $this->setUser($user_id);
    }
    public function setUser($user_id) {
        $user = $this->db->query("SELECT * FROM users WHERE id='".$user_id."' LIMIT 1");
        if($user->num_rows >= 1) {
            $user = $user->fetch_object();
            $this->user = $user;
        }
    }

and in index.php just$user = new User($_SESSION['user_id'],$db,$domain);

回答3:

It is not set a parent constructor after inherit.

for example

class Core {
        public $db;
        public $domain;
        public $settings;
        public function getDomain() {
            return $this->domain;
        }
        function __construct($db,$domain) {
            $this->db = $db;
            $this->domain = $domain;
            $this->getSettings();
        }
        public function getSettings() {
            $settings = $this->db->query("SELECT * FROM settings");
            $settings = $settings->fetch_object();
            $this->settings = $settings;
        }
}

class User extends Core 
{
    public $user;

    // parent:param Parameters add 
    function __construct($user_id,$db,$domain) {
        // parent constructer call 
        parent::__construct($db,$domain);
        $this->setUser($user_id);
    }
    public function setUser($user_id) {
        $user = $this->db->query("SELECT * FROM users WHERE id='".$user_id."' LIMIT 1");
        if($user->num_rows >= 1) {
            $user = $user->fetch_object();
            $this->user = $user;
        }
    }
}


$user = new User($_SESSION['user_id'],$db,$domain);

Is this not okay? http://php.net/manual/en/language.oop5.decon.php