I'm trying to run these command from code, but having some trouble finding out how.
php composer.phar dump-autoload -o
php composer.phar require 'vendor/some-package'
php composer.phar update 'vendor/some-package'
EDIT
Forgot to mention, that I don't want to use the exec function because of potential risk and the fact that's blocked on most servers by default.
I did manage to download the .phar file from code, but not how to run these commands. I'm trying to build a friendly as possible solution and don't want use to dumpautoload manually after installing something etc.
Code to download
$composerPath = base_path('composer.phar');
copy('https://getcomposer.org/composer.phar', $composerPath);