My version of signup-controller.classes.php from youtube video https://www.youtube.com/watch?v=BaEm2

<?php

class SignupController extends Signup {

    private $username;
    private $password;
    private $repeat_password;
    private $email;

    public function __construct($username, $password, $repeat_password, $email)
    {
        $this->username = $username;
        $this->password = $password;
        $this->repeat_password = $repeat_password;
        $this->email = $email;
    }

    public function signup_user()
    {
        if($this->empty_input() == false) {
            // echo "Empty Input!";
            header("location: ../index.php?error=emptyinput");
            exit();
        }

        if($this->invalid_username() == false) {
            // echo "Invalid username!";
            header("location: ../index.php?error=invalidusername");
            exit();
        }

        if($this->invalid_email() == false) {
            // echo "Invalid Email!";
            header("location: ../index.php?error=invalidemail");
            exit();
        }

        if($this->password_match() == false) {
            // echo "Password Mismatch!";
            header("location: ../index.php?error=passwordmismatch");
            exit();
        }

        if($this->username_taken() == false) {
            // echo "username or email already exists";
            header("location: ../index.php?error=unknownusername");
            exit();
        }

        $this->set_user($this->username, $this->password, $this->email);
    }

    private function empty_input()
    {
        $result = true;

        if(empty($this->username) || empty($this->password) ||
           empty($this->repeat_password) || empty($this->email))
        {
            $result = false;
        }

        return $result;
    }

    private function invalid_username()
    {
        $result = true;

        if(!preg_match("/^[a-zA-Z0-9]*$/", $this->username))
        {
            $result = false;
        }

        return $result;
    }

    private function invalid_email()
    {
        $result  = true;

        if(!filter_var($this->email, FILTER_VALIDATE_EMAIL))
        {
            $result = false;
        }

        return $result;
    }

    private function password_match()
    {
        $result = true;

        if($this->password !== $this->repeat_password)
        {
            $result = false;
        }

        return $result;
    }

    private function username_taken(){
        $result = true;

        if(!$this->check_user($this->username, $this->email))
        {
            $result = false;
        }

        return $result;
    }

}