command whoami and echo $USER are giving different outputs how is that even possible?

If I don't fix this I can't properly run bash scripts that checks whether the $USER is root in an IF statement, because, as it is, the $USER is never root, even when the whoami is the root. No matter if I su to root, I keep being whatever user I was before, at least for the $USER variable. How do I fix this? I have tried setting the variable $USER to be equal to the output of the command whoami on the .zshrc file, but it didnt work at all.