require is working but require_once is not - php

Question

Routes.php
use MyMVC\Core\Route;
$route = new Route;
$route->add('/', 'HomeController#index');
$route->add('about', 'AboutController#index');
$route->add('contact', 'ContactController#index');
Index.php
<?php
/**
* Define Constants
*/
define('BASE_PATH', dirname(realpath(__FILE__)));
define('APP_PATH', BASE_PATH . "/app");
/**
* Including the Composer's autoloader
*/
require_once 'vendor/autoload.php';
/**
* Load the routes declarations
*/
require_once 'app/routes.php';
/**
* Bootstrap our application
*/
require_once 'app/init.php';
/**
* Initialize our beautiful framework
*/
$application = new \MyMVC\Application($route);
composer.json
"autoload" : {
"psr-4" : {
"MyMVC\\" : "app/"
},
"classmap": [
"app/Controllers",
"app/Helpers"
],
"files": ['app/routes.php'] // already removed this line
},
When using require_once it is giving undefined variable route error while if i use only require it shows the route object.
Why is that so ?

Answer 1

This kind of error only happens when the file is already loaded from other files. That's why it's not loading it again and you are not getting instance of $route object.
E.g,
Let's say
file1.php included routes.php
Now if you use:
require 'routes.php' (it will load same file again even already loaded)
requier_once 'routes.php' (it will not load the file if already loaded, and as you are not getting instance of $route variable, it's mean it's happening)

Answer 2

All symptoms suggest that 'app/routes.php' is included somewhere else before. Since it defines a variable, if such include does not happen in global scope the variable will be local to wherever it's called from.
Apart from using a dedicated debugger like Xdebug, you can use builtin tools to diagnose the issue. For instance, you have get_included_files() to get a list of included files in a given point. You can also add debug_print_backtrace() on top of 'app/routes.php' to find out where it's called from.
Note on updated question and follow-up comment: if you're trying to auto-load the file and the file gets loaded automatically, I'd say you've just answered your own question. But it's worth noting that auto-loading is intended to be used on functions and class definitions. You have an arbitrary code snippet that defines a variable and —as you've just learnt— since the variable becomes local to the auto-loader method it isn't of much use.