Database Constructor PHP Tutorial

<?php
/**
 * Simple PHP Class That interacts with a database object
 */

/**
 * This is the database wrapper we will use to interact with the database object
 * known as `PDO`.
 */
class Database
{
    /**
     * This is the database variable that stores are PDO object.
     * The PDO object is the main object that will communicate to the database server.
     * I made the database object protected, as this allows/forces me to write clean,
     * and safe code, by forcing all database logic to be executed from the Database
     * class itself.
     *
     * The variable name is not important, it could be pdo, or instance.
     */
    protected $database;

    /**
     * This is the database constructor that will initialize are connection
     * to the database server.
     *
     * @param hostname. This is the hostname/ip address/domain the mysql server is located at.
     * @param username. This is the username that is allowed to connect to the database server.
     * @param password. This is the password for the username.
     * @param database. This is the name of the database we wanna connect to and use.
     */
    public function __construct($hostname = '127.0.0.1', $username = 'guest', $password = '', $database = 'testdb')
    {
        /**
         * The PDO driver expects a dns string that tells the driver what database
         * server we are using, the database name, and the hostname itself.
         *
         * PHP Strings can append other strings with . seperating the strings.
         */
        $dns = 'mysql:dbname=' . $database . ';host=' . $hostname;

        /**
         * Another way we can achievement this is with using a function called sprintf
         * which is a string formatting buffer.
         * The formated string will be 'mysql:dbname=%s;hostname=%s'.
         * The '%s' is telling sprintf that the first and second variable will be a string.
         * sprintf also allows integers, or decimals using '%d'.
         */
        $formatted_dns = sprintf('mysql:dbname=%s;host=%s', $database, $hostname);

        /**
         * Unlike the `mysqli` driver PDO will try to initialize, and if it can't it will throw an
         * exception, To catch the exception we have to use a try block in php.
         */
        try {
            /**
             * $this-> is an accessor for variables declared inside the class object/scope.
             * $this-> can access variables or functions also known as methods inside an object.
             *
             * PDO expects 3 variables at least, usually. The first being the dns string.
             * The second and third being the username and password.
             */
            $this->database = new PDO($formatted_dns, $username, $password);
        } catch (PDOException $exception) {
            /**
             * If the database could not be connected to for any reason,
             * like the server being down, or the PDO driver not being installed
             * an exception will be thrown.
             */
            echo 'The database connection failed: ' . $exception->getMessage();
        }
    }
}