failed to include header file

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PHP

I am using XAMPP and learning from online tutorials, i have only vedio not the source code

this is my directory structor

source file-classes(folder):core.php

source file-emails(folder)

source file-inc(folder):autoload.php,config.php, index.php

source file-pages(folder): index.php

source file-template(folder):_header.php;

the code for config.php

<?php

if(!isset($_SESSION)){
    session_start();
}

//site domain name with http

 defined"SITE_URL")

|| define("SITE_URL", "http://".$_SERVER['SERVER_NAME']);



//directory separator
defined("DS")|| define("DS", DIRECTORY_SEPARATOR);


// root pat

defined("ROOT_PATH")|| define("ROOT_PATH", realpath(dirname(__FILE__).DS."..".DS));


//classes folder

defined("CLASSES_DIR")|| define("CLASSES_DIR", "classes");

//defined pages

defined("PAGES_DIR")|| define("PAGES_DIR", "pages");

//module folder

defined("MOD_DIR")|| define("MOD_DIR", "mod");

//inc folder

defined("INC_DIR")|| define("INC_DIR", "inc");

//template folder

defined("TEMPLATE_DIR")|| define("TEMPLATE_DIR", "template");



//email path

defined("EMAILS_PATH")|| define("EMAILS_PATH", ROOT_PATH.DS."emails");

//catalogue images path

defined("CATALOGUE_PATH")|| define("CATALOGUE_PATH", ROOT_PATH.DS."catalogue");

//add all above directories to include path

set_include_path(implode(PATH_SEPARATOR, array(
            realpath(ROOT_PATH.DS.CLASSES_DIR),
    realpath(ROOT_PATH.DS.PAGES_DIR),
    realpath(ROOT_PATH.DS.MOD_DIR),
        realpath(ROOT_PATH.DS.INC_DIR),
        realpath(ROOT_PATH.DS.TEMPLATE_DIR),

        get_include_path()



)));
?>

and the code for autoload.php is

<?php

require_once ('config.php');
function __autoload ($class_name){
    $class = explode ("_",$class_name);
    $path=implode ("/",$class).".php";
    require_once ($path);
}

?>

code for the index.php in the inc folder is

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

$core = new Core();

$core->run();

?>

code for the pages(folder) index.php is

<?php
require_once '_header.php';
?>

when i am running this last index file it show the error,
and the error is

Warning: require_once(_header.php): failed to open stream: No such file or directory in C:\xampp\htdocs\sebastine\pages\index.php on line 5

Fatal error: require_once(): Failed opening required '_header.php' (include_path='.;C:\xampp\php\PEAR') in C:\xampp\htdocs\sebastine\pages\index.php on line 5

please some body help me what i am doing wrong my guess is that i am doing some thing wrong in the config files but i can not find it

Thanks