Error including files in php using require_once - php

Question

This is my app. root:
htdocs
core/
init.php
classes/
DB.php
POST.php
ajax/
post_feeds.php
This is my post_feeds.php;
require_once '../core/init.php';
The file init.php is there but I don't understand why I am getting this error.
Warning: require_once(classes/DB.php): failed to open stream: No such file or directory in C:\Apache24\htdocs\core\init.php on line 9
Fatal error: require_once(): Failed opening required 'classes/DB.php' (include_path='.;C:\php\pear') in C:\Apache24\htdocs\core\init.php on line 9
It gives me no error in my init.php file when I require_once 'classes/DB.php';
This is init.php
session_start();
date_default_timezone_set('Etc/GMT+2');
//date_default_timezone_set('Europe/Tirane');
//autoload for classes.
require_once 'classes/DB.php';
require_once 'classes/USER.php';
require_once 'classes/NOTIFICATION.php';
if (isset($_COOKIE["user_id"])) {
$_SESSION['user_id'] = $_COOKIE['user_id'];
}

Answer 1

Try to use an absolute path for including files:
$root = realpath($_SERVER["DOCUMENT_ROOT"]);
require_once("{$root}/core/init.php");
Or with dirname:
require_once(dirname(__FILE__) .'/../core/init.php');

Answer 2

You dont have problem with including init.php, you have problem with init.php on line 9:
Warning: require_once(classes/DB.php): failed to open stream: No such file or directory in C:\Apache24\htdocs\core\init.php on line 9
Open init.php and change your path on line 9 from require_once('classes/DB.php') to require_once('../classes/DB.php').
DB.php is in classes folder in your root, and in line 9 on init.php you are including ROOT/core/classes/DB.php.