PHP OOP - Database class (Undefined variable)

Question

Learning OOP with PHP I have created a database class, but I am getting undefined variable db_host db_name db_username and db_password. I've tried with $this->db_host which removes the undefined variable for db_host but then I get a Fatal error saying "Using $this when not on object context".

<?php
class database
{
    private $db_host = "";
    private $db_username = "";
    private $db_password = "";
    private $db_name = "";

    static function connect()
    {
        try {
            new PDO("mysql:host=" . $db_host . '; dbname=' . $db_name, $db_username, $db_password);
            setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        } catch (PDOException $db_error) {
            echo $db_error->getMessage();
        }
    }
}
?>

Answer 1

You cannot access non static instance fields from static functions (but you can access static instance fields and functions from a non static method). Moreover, you are misusing " setAttribute " function.

You should either make your instance variables static and use self keyword:

class database
{
    private static $db_host = "...";
    private static $db_username = "...";
    private static $db_password = "...";
    private static $db_name = "...";

    public static function connect()
    {
        try {
            $dbConnection = new PDO("mysql:host=" . self::$db_host . '; dbname=' . self::$db_name, self::$db_username, self::$db_password);
            $dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        } catch (PDOException $db_error) {
            echo $db_error->getMessage();
        }
    }
}

Or, make your function non static and relate to the class variables with $this keyword:

class database
{
    private $db_host = "...";
    private $db_username = "...";
    private $db_password = "...";
    private $db_name = "...";

    public function connect()
    {
        try {
            $dbConnection = new PDO("mysql:host=" . $this->db_host . '; dbname=' . $this->db_name, $this->db_username, $this->db_password);
            $dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        } catch (PDOException $db_error) {
            echo $db_error->getMessage();
        }
    }
}

What will you do with the "dbConnection" variable? Right now it doesn't serve any purpose. You should return it, or bind another instance field with it.