Php oturum açma ...

Merhaba arkadaşlar, session kullanarak giriş yapıyordum şimdiye kadar ama artık oturum bilgilerini ömür boyu çerezde tutmak istiyorum ne denediysem olmadı neleri nasıl değiştirmeliyim ne eklemeliyim? yardım edin lütfen !

//register.php
<?php
require_once("includes/config.php");
require_once("includes/classes/FormSanitizer.php");
require_once("includes/classes/Constants.php");
require_once("includes/classes/Account.php");


    $account = new Account($con);

    if(isset($_POST["submitButton"])) {
        
        $firstName = FormSanitizer::sanitizeFormString($_POST["firstName"]);
        $lastName = FormSanitizer::sanitizeFormString($_POST["lastName"]);
        $username = FormSanitizer::sanitizeFormUsername($_POST["username"]);
        $email = FormSanitizer::sanitizeFormEmail($_POST["email"]);
        $email2 = FormSanitizer::sanitizeFormEmail($_POST["email2"]);
        $password = FormSanitizer::sanitizeFormPassword($_POST["password"]);
        $password2 = FormSanitizer::sanitizeFormPassword($_POST["password2"]);
        
        $success = $account->register($firstName, $lastName, $username, $email, $email2, $password, $password2);

        if($success) {
            $_SESSION["userLoggedIn"] = $username;
            header("Location: index.php");
        }
    }

function getInputValue($name) {
    if(isset($_POST[$name])) {
        echo $_POST[$name];
    }
}  

//login.php
<?php
require_once("includes/config.php");
require_once("includes/classes/FormSanitizer.php");
require_once("includes/classes/Constants.php");
require_once("includes/classes/Account.php");

$account = new Account($con);

    if(isset($_POST["submitButton"])) {

        $username = FormSanitizer::sanitizeFormUsername($_POST["username"]);
        $password = FormSanitizer::sanitizeFormPassword($_POST["password"]);
        
        $success = $account->login($username, $password);

        if($success) {
            $_SESSION["userLoggedIn"] = $username;
            header("Location: index.php");
        }
    }

function getInputValue($name) {
    if(isset($_POST[$name])) {
        echo $_POST[$name];
    }
}  
?>

//config.php

<?php
ob_start(); 
session_start();

//index.php
if(!isset($_SESSION["userLoggedIn"])) {
    header("Location: giris.php");
}

$userLoggedIn = $_SESSION["userLoggedIn"];