if ($_SERVER["REQUEST_METHOD"] == "POST") {

    //two passwords are equal to each other
    if ($_POST['password'] == $_POST['confirmpassword']) {

        //define other variables with submitted values from $_POST
        $username = $mysqli->real_escape_string($_POST['username']);
        $email = $mysqli->real_escape_string($_POST['email']);

        //md5 hash password for security
        $password = md5($_POST['password']);

        //path were our avatar image will be stored
        $avatar_path = $mysqli->real_escape_string('images/'.$_FILES['avatar']['name']);

        //make sure the file type is image
        if (preg_match("!image!",$_FILES['avatar']['type'])) {

            //copy image to images/ folder
            if (copy($_FILES['avatar']['tmp_name'], $avatar_path)){

                //set session variables to display on welcome page
                $_SESSION['username'] = $username;
                $_SESSION['avatar'] = $avatar_path;

                //insert user data into database
                $sql =
                "INSERT INTO users (username, email, password, avatar) "
                . "VALUES ('$username', '$email', '$password', '$avatar_path')";

                //check if mysql query is successful
                if ($mysqli->query($sql) === true){
                    $_SESSION['message'] = "Registration successful!"
                    . "Added $username to the database!";
                    //redirect the user to welcome.php
                    header("location: welcome.php");
                }
            }
        }
    }
}