Database Singleton Error

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();
}
?>

you should use _only_ composition instead of inheritance. Do not inherit user from DB class (you're using composition already) — Ejaz, May 13 '14 at 23:58
Now i'm getting this Error Fatal error: Call to undefined method DB::query() in C:\xampp\htdocs\docs\users.php on line 11 — Zayn Ali, May 14 '14 at 00:01
yes but i'm trying to get the mysqli object through DB constructor so that i can use it outside the class like in user.php. how to do this? — Zayn Ali, May 14 '14 at 00:08
(Singletons are so bad. What if you have 2 dbs later on? If you want 1 of something, just make 1. Don't use a crappy design pattern that'll bite you in the butt 100.000 LOC later. Just my humble opinion.) — Rudie, May 14 '14 at 00:25

Ejaz · Answer 1 · 2014-05-14T08:45:57.560

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);
     }
  }

Thanks Ejay for answering. I had made this code before, i's just trying to do it with a constructor and a typical method. Can u demostrate it with a constructor method? — Zayn Ali, May 14 '14 at 01:57

Database Singleton Error

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