Unable to access variable from base .php called by required php - php

Question

assume i have a code as below:
index.php
<?php
require 'file_1.php';
// file_1.php contents can go here, but i'm trying to make this simple and clutter free, thus clean.
// The reason being to not centralize all code in a single file should file_2.php, file_3.php, file_n.php exist.
$app = new type_of_object();
$app->intialize_some_stuff();
?>
file_1.php
<?php
$app->access_some_method($param1, new function($app) {
// do some logic here...
});
?>
doing this approach, i get an error such as below:
Notice: Undefined variable: app in X:\project_folder\file_1.php on line 2
to summarize, i'm trying to decentralize all codes from being defined in index.php and instead, have them hosted by different files such as file_1.php to file_n.php. The problem is, i'd be needing to access the $app variable from index.php on the required files such as file_1.php to file_n.php since i'll only need 1 instance of this, no duplicates allowed.
i'm also open to being taught another approach on this.
cheers!
EDIT: its worth nothing that i'm fairly new in PHP, and that i've already done few research. except that what i'm getting is the opposite of my issue. also, the need to access $app of index.php from other php files is the very center of my question.

Answer 1

Splitting code can be a good idea to achieve a better maintainability. You should avoid unclear scripts name.
at a high degree, you want definitely go on composer approach to maintain packages.
Anyway, for a basic code splitting, the rules are the same than a regular script. Code is interpreted at the time it is read. Just rewrite your example in one script :
<?php
$app->access_some_method($param1, new function($app) {
// do some logic here...
});
$app = new type_of_object();
$app->intialize_some_stuff();
?>
See the problem ? $app is simply not initialized when calling the access_some_method.
So, this will work :
<?php
$app = new type_of_object();
$app->intialize_some_stuff();
require 'file_1.php';
?>
But the gain is poor. If you already have a class, there's a few cases where it can be interesting to split logic among many files. I think this one is a better example (all files in the same folder, no namespaces) :
<?php
require('App');
require('AppRequest');
$app = new App();
$request = new AppRequest();
$app->process($request);
With this way, you can split the main app logic and the request analysis logic for instance