Using connection method constructed in other class doesn't work - php

Question

I have been recently toying with dynamic PDO connection methods. However I run into some problems when I started to use other classes.
Why can't I connect to database in Admin class with method that has been constructed in Server class?
I have tried many solutions. This one seemed most logical to me ...
How do I make it work so that I wouldn't have to construct connection in every class?
class Server
{
private $hostdb = 'blah';
private $namedb = 'blah';
private $userdb = 'blah';
private $passdb = 'blah';
public static $conn;
public $errorMessage = 'If you read this text, contact web administrator and tell him about your problem.';
public function __construct()
{
$this->connect();
}
public function connect()
{
try {
$this->conn = new PDO("mysql:host=$this->hostdb; dbname=$this->namedb", $this->userdb, $this->passdb, array(PDO::ATTR_PERSISTENT => true));
$this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$this->conn->exec("SET CHARACTER SET utf8");
} catch (PDOException $e) {
echo 'Connection failed: ' . $e->getMessage();
}
}
}
And the Admin class:
class Admin extends User
{
function someFunction($table)
{
try {
$sql = "SELECT * FROM $table";
//I want to change this line so that my connection would work
$result = Server::$conn->query($sql);
while ($row = $result->fetch(PDO::FETCH_NUM)) {
//Do something
}
} catch (PDOException $e) {
//Show when debugging
//echo $e->getMessage();
echo Server::errorMessage;
}
}
}
I have instantiated both Server, User and Admin class in config.req.php file.
When I changed "Server::$conn->" to "static::$conn->" it still gave me an error.

Answer 1

Make sure you have instantiated Server at least once in the Request. Otherwise, connect() will never be called. Also, $this->conn will create a new public instance property. Static properties need to be set with static::$conn or self::$conn.
So change your connect() method to
public function connect()
{
try {
self::$conn = new PDO("arguments …"));
self::$conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
self::$conn->exec("SET CHARACTER SET utf8");
// … shortened for brevity
On a side note, why not just use Dependency Injection? That makes for more maintainable and testable designs, e.g. keep the $this->conn and remove all the static stuff instead (in addition to the constructor) for something like
class Server
{
private $hostdb = 'blah';
private $namedb = 'blah';
private $userdb = 'blah';
private $passdb = 'blah';
private $conn;
public function getConnection()
{
if (!isset($this->conn)) {
$this->connect();
}
return $this->connection;
}
And then for your Admin class:
class Admin extends User
{
private $server;
public function __construct(Server $server)
{
$this->server = $server;
}
function someFunction($table)
{
try {
$sql = "SELECT * FROM $table";
$result = $this->server->getConnection()->query($sql);
On another side note:
$sql = "SELECT * FROM $table"
Interpolating strings into a query makes you open to SQL Injection attacks. Use prepared statements instead.