PDO database connection can not be used in other classes, but others can?

Question

I've been working on a databaseHandler php class, which should connect to the database and then be usable in other PHP files, atleast this was the plan. I've come across the problem that it cannot use any PDO related functions in my PHP class, i've tried checking if it was null, or not set at all (but it was) and i've also tried using a dummy function that just echos something which was to test if the class isn't undefined in others.

<?php
class databaseHandler {

    private $db;
    private static $instance;

     function __construct() {
        $this->buildDatabaseConnection();
    }

    public static function getInstance() {
        if(!self::$instance) {
            self::$instance = new databaseHandler();
        }

        return self::$instance;
    }

    private function buildDatabaseConnection() {
        require 'dbconfig.php';
        try {
    $this->db = new PDO("mysql:host=" . HOST . ";dbname=" . DATABASE . ";charset=utf8", USER, PASSWORD);
    if(isset($this->db)) {
        if(!is_null($this->db)) {
            echo "isn't null";
        }
    }
    } catch(PDOException $e) {
        $e->getMessage();
    }
    }

    public function getConnection() {
        return $this->$db;
    }

    public function getSomeShit() {
        echo "some shit";
    }


}
?>

The problem might be with the getConnection() php function, does it actually return the PDO object here? Because the main problem lays with the fact that i get this error when i use my getConnection function in other classes:

Notice: Undefined variable: database in     F:\Websites\DevProject\php\functions.php on line 69

Fatal error: Uncaught Error: Call to a member function prepare() on null in F:\Websites\DevProject\php\functions.php:69 Stack trace: #0 F:\Websites\DevProject\php\functions.php(51): register('qwe', 'sdasd', 'asdasd', 'asdas', '01-01-1970') #1 F:\Websites\DevProject\register.php(123): datelessRegister('qwe', 'sdasd', 'asdasd', 'asdas') #2 {main} thrown in F:\Websites\DevProject\php\functions.php on line 69

and line 69 would be this:

$stmnt = $database->prepare("SELECT $username, $email FROM " . USERTABLE);

Where the $database variable is the class instance:

$database = databaseHandler::getInstance();

If i try to var_dump it in my registration php file, it'll tell me this:

object(databaseHandler)#1 (1) { ["db":"databaseHandler":private]=> object(PDO)#2 (0) { } }

However, when i use it inside of my function, it will say it is 'NULL', why is it null in a function but defined globally?

All the errors relate to the object, and in there it tells me the pdo is undefined (i've tried checking if it was null, but it wasn't!)

Any help would be great, thanks in advance.

Answer 1

In the return statement of getConnection method you should write the class variable without $:

return $this->db;

Also, you must do the queries on the PDO connection instead of your database handler class, so $database = databaseHandler::getInstance(); should be

$database = databaseHandler::getInstance()->getConnection();

or you can simply use the __call magic method to keep your code as it is right now:

public function __call($method, $args)
{
    return call_user_func_array(array($this->db, $method), $args);
}

Also, to clarify how to use this class, you should call it inside every method you need the database to be used, not only once on the top of your file, because defining it there it will not be available inside your methods/functions unless you don't use the global statement to import the variable. However usage of the global variable is not encouraged and it is considered as a bad practice. An example on how to use the database would be:

public function doSomething()
{
    $db = databaseHandler::getInstance()->getConnection();
    $db->prepare(....);
}

public function doSomethingElse()
{
    $db = databaseHandler::getInstance()->getConnection();
    $db->prepare(....);
}

If you know there are many methods inside your class that uses the database then you can define it as a class variable and set the db instance in the constructor.

Answer 2

The problem seems to be in the getConnection() function. Since $db is field you must do.

return $this->db;

However this is a very complex class structure for a simple problem. You can directly extend your class from PDO

class DatabaseHandler extends PDO {
    function __construct() {
        parent::__construct('mysql:host=host_name;dbname=db_name', 'user_name', 'password');
        parent::setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }
}

And create an object of this whenever you want to do database transaction.

$db = new DatabaseHandler();
$stmt = $db->prepare(".....");
$stmt->execute();

Answer 3

You seem to be having issues with scopes.

$database = databaseHandler::getInstance();

function doSomething()
{
    // $database is not defined in this scope
    $database->getConnection()->prepare(/* ... */);
}

You have to declare it as part of the global scope

function doSomething()
{
    global $database;
    $database->getConnection()->prepare(/* ... */);
}