Db connect error in abstract class - anybody help me to find the bug?

Anybody help me to find what is wrong with the below code? I have class student.php extends db.php.

Student.php

<?php
require_once('db.php');

class Student extends Db {
    
    public function __construct() {
        $res = $this->db_connect();
    }
}


$result = new Student();

db.php

<?php 
abstract class Db {
    protected $db_host;
    protected $db_user;
    protected $db_password;
    protected $db_name;
    public function __construct() {
        $this->db_host = 'localhost';
        $this->db_user = 'root';
        $this->db_password = '';
        $this->db_name = 'student';
    }
    protected function db_connect() {
        if(!isset($GLOBAL['db_connection'])) {
            $GLOBAL['db_connection'] = new mysqli($this->db_host, $this->db_user, $this->db_password, $this->db_name);
        }
        
        if(mysqli_connect_errno()) {
            $responseArray['status'] = '500';
            $responseArray['response'] = 'Error'. mysqli_connect_error().' Error No: '. mysqli_connect_errno();
        }
        else {
            $responseArray['status'] = '200';
            $responseArray['response'] = 'Database connection success';
        }
        
        return $responseArray;
    }
    
}

?>

it returns the result

mysqli::__construct(): (HY000/1045): Access denied for user ''@'localhost' (using password: NO)

I understand DB variables are empty that's why I get the error. but I need to make sure what's wrong with the code.

You have override __construct() method so default values are not set. Check this:

<?php
require_once('db.php');

class Student extends Db {
    
    public function __construct() {
        // call Db's constructor
        parent::__construct();

        $res = $this->db_connect();
    }
}