Call to a member function on a non-object - php

Question

I am new to php oop , so struggling a bit .
i have a database connection class viz :
class config{
protected $HOST = "localhost";
protected $USERNAME = "something" ;
protected $PASSWORD = "something";
protected $DATABASE = "something";
// Constructor - open DB connection
function __construct() {
try {
$this->db = new mysqli($this->HOST, $this->USERNAME, $this->PASSWORD, $this-
>DATABASE);
$this->db->autocommit(FALSE);
}
catch(Exception $e)
{
if($this->db->connect_errno > 0){
echo 'Caught exception: ', $e->getMessage(), "\n";
}
}
}
// Destructor - close DB connection
function __destruct() {
$this->db->close();
}
}
$api = new Config();
Now i have another class from which i need to perform some tasks ...but i get FATAL error .
second class :
class Myclass extends config {
function __construct(){}
public function myfunction()
{
try{
$stmt = $this->db->stmt_init(); /* Error here : Fatal error: Call to a member
function stmt_init() on a non-object */
$query = "SELECT ABC FROM table " ;
$stmt = $this->db->prepare($query); /* Error here : Fatal error: Call to a member
function prepare() on a non-object */
}
catch(){}
}
}
Please guide me with proper remedial code snippet

Answer 1

In your child class, you need to call the parent constructor. PHP won't automatically call parent constructor's when a child class is instantiated.
class Myclass extends config {
function __construct($h, $u, $p, $d){ parent::__construct($h, $u, $p, $d); }
Also, you don't have a $db property in the parent class, so add that
class config{
protected $db;
protected $HOST = "localhost";
protected $USERNAME = "something" ;
protected $PASSWORD = "something";
protected $DATABASE = "something";
Edit: Dependency Injection approach:
class config{
public $HOST = "localhost";
public $USERNAME = "something" ;
public $PASSWORD = "something";
public $DATABASE = "something";
}
class Myclass
{
protected $db;
function __construct($db)
{
$this->db = $db;
}
public function myfunction()
{
// do whatever with $this->db
}
}
$config = new Config();
try
{
$db = new mysqli($config->HOST, $config->USERNAME, $config->PASSWORD, $config->DATABASE);
$db->autocommit(FALSE);
}
catch(Exception $e)
{
if($db->connect_errno > 0){
echo 'Caught exception: ', $e->getMessage(), "\n";
}
}
$myclass = new Myclass($db);
Have a look into Dependency Injection. This is a favorable approach for dealing with classes that need access to a database. Instead of having all your classes extend the database, just pass the datsbase object as a parameter when creating new classes (that need the db).

Answer 2

Your derived class constructor is not calling the base constructor, so $this->db does not have the value it's supposed to. In PHP you must do this explicitly.
In this particular case, you should remove the derived constructor altogether since it's not doing anything. This will let PHP use the base constructor directly.