Database Singleton Error - php

Question

I'm getting this Error by using this singleton class. Please explain me what i'm doing wrong in that? i'm trying to get the sqli object through constructor.
Fatal error: Call to private DB::__construct() from invalid context in C:\xampp\htdocs\docs\users.php on line 20
<?php
class DB
{
#############################
##### Define Connection ####
#############################
const HOST = 'localhost';
const USER = 'root';
const PASSWORD = '';
const DATABASE = 'ali';
#############################
// Data Member For MySQLi Database Connection
private static $mysqli = NULL;
// Database Connection
private function __construct()
{
#self::$mysqli = new mysqli(self::HOST, self::USER, self::PASSWORD, self::DATABASE);
if (self::$mysqli->connect_error) {
throw new Exception("<b>Database Connect Error: </b>" . self::$mysqli->connect_error);
}
if (self::$mysqli->error) {
throw new Exception("<b>Database Select Error: </b>" . self::$mysqli->error);
}
echo "<span style='color: green; font-size: 18px; font-weight: bold'>Connected</span>
<br>"; // For testing //
return self::$mysqli;
}
// Class Level Static Function to Get Database Connection
public static function get_connection()
{
if ( ! isset(self::$mysqli) )
{
self::$mysqli = new self();
}
return self::$mysqli;
}
}
?>
my 2nd file where i'm using this class
<?php
require_once 'DB.php';
class users extends DB
{
public function get_results()
{
$sqli = DB::get_connection();
$user_data = $sqli->query("SELECT * FROM `users`");
while($row = $user_data->fetch_object())
{
echo $row->name . "<br>";
}
}
}
$user = new users;
try {
$user->get_results();
} catch (Exception $e) {
echo $e->getMessage();
}
?>

Answer 1

if you're not aiming to build that typical database connection singleton then following code will achieve what you want
class DB
{
const HOST = 'localhost';
const USER = 'root';
const PASSWORD = '';
const DATABASE = 'yourdb';
private static $mysqli = null;
private function __construct() {
//an empty private constructor to prevent creating instances of this class
}
public static function get_connection() {
if(!self::$mysqli) {
//create mysqli object here and return instead of (typically returned) self instance
$conn = new mysqli(self::HOST, self::USER, self::PASSWORD, self::DATABASE);
if($conn->connect_error) {
throw new Exception("<b>Database Connect Error: </b>" . $conn->connect_error);
}
if($conn->error) {
throw new Exception("<b>Database Select Error: </b>" . $conn->error);
}
self::$mysqli = $conn;
}
return self::$mysqli;
}
}
user class doesn't need any changes, except for removing extends DB, to eliminate the error.
Update
class DB
{
const HOST = 'localhost';
const USER = 'root';
const PASSWORD = '';
const DATABASE = 'yourdb';
private $mysqli = null;
private function __construct() {
$conn = new mysqli(self::HOST, self::USER, self::PASSWORD, self::DATABASE);
if($conn->connect_error) {
throw new Exception("<b>Database Connect Error: </b>" . $conn->connect_error);
}
if($conn->error) {
throw new Exception("<b>Database Select Error: </b>" . $conn->error);
}
$this->mysqli = $conn;
}
/**
* #return DB
*/
public static function get_connection() {
/**
* #static $DB DB a static variable that is not class member but will be _shared_ across all calls to DB::get_connection()
*
* WTF? http://stackoverflow.com/questions/6601027/php-and-static-variables-in-object-member-functions
*/
static $DB = null;
if($DB == null) {
$DB = new DB();
}
return $DB;
}
/**
* A wrapper for internal database object/connection/resource's query method
* #param $sql String the sql query
*
* #return bool|mysqli_result
*/
public function query($sql) {
return $this->mysqli->query($sql);
}
}