In TAO - there are several modules that use constants.php and you may need to reference these to get certain pieces of code to work properly.

Step 1 - Create an includes folder in your module

As explained, navigate to your module and create a folder called includes

Step 2 - Create a file in the includes folder called constants.php

You should now be able to see a file called
\includes\
constants.php

Step 3 - Add the following code

This is a sample to import the constants for TAO results