How do I load a class in Vendor folder ?
I am trying to load this file :
my_site/vendor/payplug/lib/init.php
I am updating from cake 2.x to 3.3.7
Was it installed with composer? If it’s a namespaced class, then it should be handled by the autoloader (composer dumpautoload to refresh that), and you can use it the same as any other class.
Was not 
Installed old school !
You may need to add details in your own composer.json, then, to tell it about the namespace in that folder. See Manually Installing a Plugin.
OK. Thank you.
You are always a great help !