PHP - Composer.json Files For Installing Apps Automatically

Dear Friends,

I have 3 different Applications under the same server.
http://myserver/App1, http://myserver/App2 and http://myserver/App3

Please, could you help me to share info between them?

App1 is a "User Admin Application". Here the user should do the login. After the login, the User should be able to open App2 or/and App3
I have tried with SESSION and COOKIES. As I am under the same server, I think, I am doing something wrong. Or SESSION and/or COOKIES doesn't will work?

Thank you.

php74 is installed as my primary php, but I am also working with a different framework that can only use php73 and set up a separate pool to deal with it.  Recently, my installed composer1 told me I should upgrade to composer2 and I did so, but then found that the previously mentioned other framework also doesn't work with composer2 but only composer1.

When updating a package with composer, sometimes I get composer errors or even worse no composer errors but PHP errors later on where classes don't exist, and I think it relates to using the wrong version of PHP (and maybe even composer).

Is it always required to consistently use composer with a single version of PHP?  Anything to worry about different versions of composer or will I just get a message.  Any other best practices?  Any issues how I set up composer below?

Thanks!

curl -sS https://getcomposer.org/installer | php
# or if desired, use resulting composer.phar as: $ php (or php73) composer.phar update/install/etc
chmod +x composer.phar
sudo mv composer.phar /usr/local/bin/composer
sudo ln -s /usr/local/bin/composer  /usr/local/bin/composer2

curl -sS https://getcomposer.org/installer | php73
chmod +x composer.phar
sudo mv composer.phar /usr/local/bin/composer2_73

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php -r "if (hash_file('sha384', 'composer-setup.php') === '756890a4488ce9024fc62c56153228907f1545c228516cbf63f885e036d37e9a59d27d63f46af1d4d07ee0f76181c7d3') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;"

php composer-setup.php --version=1.10.19
chmod +x composer.phar
sudo mv composer.phar /usr/local/bin/composer1

php73 composer-setup.php --version=1.10.19
chmod +x composer.phar
sudo mv composer.phar /usr/local/bin/composer1_73

rm composer-setup.php

I've only seen composer packages using vendor name and package name namespace.

Is it possible to use deeper namespace such as the following?

For instance, package:

{
    "name": "notioncommotion/my-sub-namespace/my-package",
    "description": "MyPackage located in MySubNamespace.",
    ...
}

is used in some project:

{
    "require": {
        "notioncommotion/my-sub-namespace/my-package": "^1.2"
    }
}

If not, what do you think were the reasons composer does not allow?

Thanks

/root
  |-- /src
        |-- /App
              |-- /DBConfig
              |-- /Controller
              |-- /Model
        |-- /Core
              |-- /Helper
              |-- /Controller
              |-- /Model

"autoload" : {
    "psr-X" : {
        "App\\" : "/src",
        "Core\\" : "/src"
    }
}

Quick question about the dependency manager Composer with PHP. I have it installed and am pulling in a few libraries, it works pretty well as far as I can tell. I have a question that might be dumb, so I apologize in advance.

When connecting to your application it's still required to use the lines similar to the following?

<script src="vendor/components/jquery/jquery.min.js"></script>
<link rel="stylesheet" href="vendor/twbs/bootstrap/dist/css/bootstrap.css" >

Also I'm trying to figure out how to install the x3dom library, but cannot get composer to do it. (https://www.x3dom.org/)