How to create custom variables in the wp-config

I would like to add some custom config variables in the wp-config.php file (not in the functions.php). Then, I need to access to these variables in my templates.
What is the way to do that ?

Why do you need this?

In wp-config you can set defines. They will be accesible everywhere without needing to use globals. Define something: define('MY_DEFINE_NAME', 'THE_VALUE');.

Then in your templates you can show the value like this: echo MY_DEFINE_NAME;

Or set the value to a variable: $var = MY_DEFINE_NAME;.