Error while retrieving a record from database using PHP and MySQL

Question

I'm a beginner in PHP. I'm trying to get data from database but I'm having the following error "Fatal error: Call to a member function query() on a non-object in C:\xampp\htdocs\phpExamPrep\studentDB.php on line 22" I'm sorry I have to include all the codes, its just to help you help me. Thanks I have the following classes

class database 
{
    private static $dsn = 'mysql:local=localhost;dbname=test1';
    private static $username ='me';
    private static $password ='me';
    public static $db;

    private function __construct() 
    {

    }

    public static function  dataConnect()
    {
        if(isset(self::$db))
        {
           try
           {
               self::$db = new PDO(self::$dsn, self::$username, self::$password);

           }
           catch(PDOException $e)
           {
                $error_message = $e->getMessage();
                include 'database_error.php';
                exit();
           }
        }
           return self::$db;
      }
   }


   class studentDB 
   {


       public static function getAllRecords()
       {
           require  'database.php';
           $query = 'select * from student';

           $db = database::dataConnect();
           $result = $db->query($query); //This part is giving error
           $students = array();

           foreach($result as $row)
           {
              $stud = new student($row['firstname'],$row['lastname']);
              $stud->setID($row['id']);
              $students[]=$stud;
           }

              return $students;
        }
     }

    <?php
        include 'studentDB.php';
        $stx =studentDB::getAllRecords();

        foreach ($stx as $r)
        {
            echo $r->getFirstName().' '.$r->getLastName().' '.$r->getID();
        }
    ?>

when I changed if(isset(self::$db)) to if(!isset(self::$db)), the error becomes 

"Warning: Invalid argument supplied for foreach() in C:\xampp\htdocs\phpExamPrep\studentDB.php on line 25"

Answer 1

I think this

if(isset(self::$db))

must be changed in

if ( ! isset(self::$db) )

as you want to set self::$db if it's not set already and not the opposite.

From the code fragment that you posted in fact, it looks like this call

$db = database::dataConnect();

is returning a null reference to $db and therefore you cannot call query on a null reference.