example

<?php
    class database
    {
        function __construct()
        {
            $this->dBusername = 'xxx';
            $this->dBpassword = 'xxx';
            $this->dBhost = 'localhost';
            $this->dBdatabase = 'xx';
            $this->dBcharset = 'utf8';
        }

        function register_account($username, $password, $email, $activation_id)
        {
            $mysqli = new mysqli($this->dBhost, $this->dBusername, $this->dBpassword, $this->dBdatabase);

            if(mysqli_connect_error())
            {
                return false;
                exit;
            }

            $sql = "INSERT INTO users (username, password, email, activation_id) VALUES (?, ?, ?, ?)";
            $stmt = $mysqli->prepare($sql);
            $stmt->bind_param("s, s, s, s", $username, $password, $email, $activation_id);
            $stmt->execute();
            $stmt->close();
            $mysqli->close();
        }

        function authenticate_account($username, $password)
        {
            // And so on
        }
    }
?>