Singleton DB connectin with configuration file

Question

Below is my code to connect which works fine without configuration file (.ini file). But if I use configuration file, I'm getting error:

Fatal error: Constant expression contains invalid operations in singletonDB.php on line 13.

But as you can see, the variables $dsn, $user and $pass are not static variables. I'm not understanding why I'm getting static variable related error for non-static variable.

My final goal is to use configuration file as well as keeping only singleton connection to DB.

<?php
$config = parse_ini_file("config.ini");
var_dump($config);
// Singleton to connect db.
class ConnectDb
{

    // Hold the class instance.
    private static $instance = null;

    private $pdo;

    private $dsn = $config['dsn_config'];

    private $user = $config['user_config'];

    private $pass = $config['password_config'];

    // The db connection is established in the private constructor.
    private function __construct()
    {
        echo nl2br("Inside constructor"); 
        $this->pdo = new PDO($this->dsn, $this->user, $this->pass);
        $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }

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

        return self::$instance;
    }

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

And this is my configuration file

;Local
dsn_config = 'mysql:host=127.0.0.1;port=3306;dbname=db_name;';
user_config = 'root';
password_config = 'root';

Thank you.

Rasclatt · Accepted Answer · 2019-09-30 06:20:20Z

1

The issue is not with $dsn, $user, or $pass, it will be with $config. You can not assign $config that way. If you change those to strings or other values (int, array, bool), you will find the error goes away:

private $dsn  = false;  # All of these are 
private $user = [];     # are valid
private $pass = 1234;   # assignments

The problem then is how do you assign the ini args? A common method is to just inject into the construct of the class when it's instantiated:

class ConnectDb
{
    private static $instance = null;

    private $pdo;    
    private $dsn = '';
    private $user = '';
    private $pass = '';

    private function __construct($dsn, $user, $pass)
    {
        $this->dns = $dns;
        $this->user = $user;
        $this->pass = $pass;

        echo nl2br("Inside constructor"); 
        $this->pdo = new PDO($this->dsn, $this->user, $this->pass);
        $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }
    ...etc.

To use:

<?php
$config = parse_ini_file("config.ini");
# Inject into the construct here
$Db = new ConnectDb($config['dsn_config'], $config['user_config'], $config['password_config']);

Another method could be using defines (constants):

<?php
$config = parse_ini_file("config.ini");
# Create some defines
define('DB_DSN', $config['dsn_config']);
define('DB_USER', $config['user_config']);
define('DB_PASS', $config['password_config']);

The class then would look like:

class ConnectDb
{
    private static $instance = null;

    private $pdo;
    private $dsn = '';
    private $user = '';
    private $pass = '';

    private function __construct()
    {
        # Assign the constants here ALTHOUGH...
        # I don't know that there is a good reason to make these class 
        # variables. I would just put the constants into the construct of
        # the PDO below. I don't know that you are going to need to reference
        # these variables after you have created the PDO connection.
        $this->dns = DB_DSN;
        $this->user = DB_USER;
        $this->pass = DB_PASS;

        echo nl2br("Inside constructor"); 
        $this->pdo = new PDO($this->dsn, $this->user, $this->pass);
        $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }
    ...etc.

edited Sep 30, 2019 at 6:20

answered Sep 30, 2019 at 5:57

Rasclatt

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2 Comments

learner Over a year ago

Thank you for the detailed explanation Rasclatt. However I didn't wanted to create the object elsewhere in my program. Hence, based on your explanation, I moved the configuration file reading code inside the constructor only. Don't know whether that is a good way of approach.

Rasclatt Over a year ago

Yeah that works too. There are benefits for doing a method like that. It’s similar to using defines in that you can create the connect anytime without having to first parse the ini and inject the array

learner · Accepted Answer · 2019-10-01 03:59:21Z

I didn't want to create object elsewhere in my program, Hence based on the explanation from @Rasclatt, I moved the configuration reading code inside the constructor and it's working fine now. Below is the updated code.

File name "singletonDB.php"

<?php

// Singleton to connect db.
class ConnectDB
{

    // Hold the class instance.
    private static $instance = null;

    private $pdo;

    private $dsn = '';

    private $user = '';

    private $pass = '';

    // The db connection is established in the private constructor.
    private function __construct()
    {
//         $pdo = new PDO($dsn, $user, $password);
        $config = parse_ini_file("../config.ini");
        echo nl2br("var_dump ing config");
        var_dump($config);

        $this->dsn = $config['dsn_config'];
        $this->user = $config['user_config'];
        $this->pass = $config['password_config'];

        echo nl2br("Inside constructor"); 
        $this->pdo = new PDO($this->dsn, $this->user, $this->pass);
        $this->pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }

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

        return self::$instance;
    }

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

Later you can get the connection form DB as

require_once 'singletonDB.php';
$instance = ConnectDB::getInstance();
$conn = $instance->getConnection();
var_dump($conn);

However I'm not sure whether it's a good way of programming.

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