How to define constants or environment variables in Magento 2.2.6?

Question

I want to define constants to use in all my project. How to define constant or environment variables to access all over the project without any special declaration ?

Answer 1

Define the constant with const keyword as :

const PATH_EMAIL_IDENTITY = 'trans_email/ident_sales';

To call constant in the same file use : self::PATH_EMAIL_IDENTITY

To call the same constant in another file use : VendorName\ModuleName\Controller\FileName::PATH_EMAIL_IDENTITY