-Create a folder in project root directory or app directory.
I am creating a folder with name Includes in App folder:(C:xampp/htdocs/ci4/App/Includes)
-Create a file CustomConstants.php, and define your constants in that file.
Save the file inside Includes folder
-use include or require method to include that file into your controllers, which contains your constants
if you want to use custom constants every where in your project, then include the file in App/Config/App.php. if your include your file here, we no need to include that file everywhere. because this location global
class App extends BaseConfig