Couldn't fetch MySQL - php

Question

I'm trying to re-code a homepage I made. This time I want to use OOP style, but I always get following error:
Statistic::checkExistingCounter() [statistic.checkexistingcounter]: Couldn't fetch MySQL
What am I doing wrong? I know that the prepare statement is senseless, but even just a query instead of prepare statement is not working at all.
Same error:
Couldn't fetch MySQL
My Database class:
class MySQL extends MySQLi {
private static $_instance = null;
private $host, $username, $password, $db;
public static function getInstance() {
if (!(self::$_instance instanceof self)) {
self::$_instance = new self();
}
return self::$_instance;
}
public function __construct(){
$this->host = '...';
$this->username = '...';
$this->password = '...';
$this->database = '...';
$this->connect();
}
public function __destruct() {
$this->db->close();
}
private function __clone(){}
public function connect() {
$this->db = #new MySQLi($this->host, $this->username, $this->password, $this->database);
/* change character set to utf8 */
$this->db->set_charset("utf8");
if (mysqli_connect_errno()) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}
return $this->db;
}
}
My statistic class:
class Statistic {
private $remote, $user_agent, $referer;
private $db;
/**
* Create Instance of MySQL
**/
function __construct($db) {
$this->db = MySQL::getInstance();
}
/**
* Check for counter today
*
* #param: string SQL
* #return: boolean (true = Counter exists, false = Counter doesnt exist)
**/
function checkExistingCounter($sql) {
$stmt = $this->db->prepare($sql);
$this->db->error;
if (!$stmt) {
echo 'Datenbankfehler';
exit;
}
$stmt->execute();
$stmt->store_result();
if ($stmt->num_rows) {
$stmt->close();
return true;
} else {
$stmt->close();
return false;
}
}
function counter() {
$sql = "SELECT ID FROM Counter WHERE Datum = CURDATE()";
$checkCounter = $this->checkExistingCounter($sql);
}
And this is a part of my index.php:
$db = new MySQL();
$statistic = new Statistic($db);
$statistic->counter();

Answer 1

You seem to be in a muddle here, implementing two sets of competing coding patterns:
your MySQL class both extends MySQLi (that is, any MySQL object is also a MySQLi object) and "delegates" to a MySQLi instance in its private variable $db
your Statistic class takes an instance of MySQL in its constructor ("dependency injection"), but then ignores it and asks the MySQL class for a "singleton" instance.
You need to read up more carefully on what each of these patterns is for, and decide on one or the other in each case (inheritance or delegation, dependency injection or singletons).
Currently, your code will do the following:
create a new MySQL object (which is also a MySQLi object, but hasn't been initialised to any particular database connection, because you haven't called parent::__construct())
in the MySQL constructor, set $this->host etc
in the connect() method, create a new MySQLi object, passing it the host etc
save this object as $this->db, which is only ever referenced in the destructor ($this->db->close())
return the MySQLi object from connect(), but nothing in __construct() is looking at that return value
back in the outer code, the MySQL object is passed to the constructor of the Statistic class
the constructor then ignores this, and calls the Singleton method MySQL::getInstance() instead
the getInstance() method (since this is the first time it has been called) will create a second MySQL object, repeating steps 1 to 5
this second MySQL object will be saved as $this->db on the Statistics object
the checkExistingCounter method attempts to use $this->db as a MySQLi connection, but the MySQL object was never connected to any database, so you get an error. (There is a connected connection, and if it wasn't private, you could access it as $this->db->db. There's another one kicking around as well, which was created at step 2, but you can't access that any more, because you ignored it at step 7.)