WordPress wp-config custom variables

I know how to add custom variables to the wp-config.php file; however, they’re not being picked up in my theme’s functions.php file. When I echo non-custom variables, they show up. When I echo custom variables, they don’t (i.e. echo SCRIP_URL echos the word SCRIP_URL versus the value in SCRIP_URL – https://test.com/). Any ideas why this is?