Avoid creating new PDO in every class - php

Question

function __construct($name, $lastName, $address, $rollNo)
{
$this->name = $name;
$this->lastName = $lastName;
$this->address = $address;
$this->rollNo = $rollNo;
$this->conn = new \PDO('mysql:host=localhost;dbname=students', 'root', '');
}
public function getUser($id){
$sql = "SELECT * FROM students WHERE id = ".intval($id);
foreach($this->conn->query($sql) as $row){
$user['name'] = $row['name'];
$user['address'] = $row['address'];
$user['roll_no'] = $row['roll_no'];
}
return $user;
}
So what i am trying to learn is how i can use a single PDO object in my all classes rather than creating $conn in all classes like users, courses etc.
I've came across the words Dependency Injection , Singleton , Factory and for me as a beginner these are all mixed up.
Also i want to know is it a bad practice what i am doing in my code for creating new objects for PDO.

Answer 1

A singleton is a control class that will ensure only one instance of a given object will exist at a time. So instead of calling $conn = new PDO() in every object, creating multiple instances of PDO(), you can write a singleton class named Database and use $conn = Database::instance() instead.
class Database
{
static $instance = null;
static function instance()
{
if (self::$instance == null)
self::$instance = new PDO('mysql:host=localhost;dbname=students', 'root', '');
return self::$instance;
}
}
Basically what it does is create a PDO() class if it doesn't exist, and reuse it if it was already instanced before.
As a side advantage, you also won't need to have your database credentials spread all over the project. The class Database will have it and noone else.

Answer 2

You could make a class called say 'Database' to deal with connecting to MySQL and setting up the PDO.
so something like...
class Database{
private $host = DB_HOST;
private $user = DB_USER;
private $pass = DB_PASS;
private $dbname = DB_NAME;
private $dbh;
private $error;
public function __construct(){
// setup dsn
$dns = 'mysql:host=' . $this->host . ';dbname=' . $this->dbname;
try{
$this->dbh = new PDO($dsn, $this->user, $this->pass);
}
catch(PDOException $e){
$this->error = $e->getMessage();
}
}
}
And then in your other classes just assign the class to a variable in your other classes so..
$db = new Database();
That would avoid your code duplication. You could also create functions in the Database class to carry out PDO queries as well.

Answer 3

Create a parent class with the connection in it and make a child class that extends the parent class
class Main {
protected $dbh;
function dbConnect() {
$options = array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION);
//DB_HOST, DB_NAME, DB_USER, DB_PASS are set via define()
//e.g. define("DB_HOST", "localhost");
$dsn = 'mysql:host=' . DB_HOST . ';dbname=' . DB_NAME;
$this->dbh = new PDO($dsn, DB_USER, DB_PASS, $options);
}
Then..
class User extends Main {
function crudRead() {
parent::dbConnect(); //called from the main class
$db = $this->dbh->prepare('SELECT * FROM user');
$db->execute();
return $db->fetchAll(PDO::FETCH_ASSOC);
}
}