How To Stop Script?

Hi, I am looking for a way to stop a script, kinda.

For example:

if(strlen($username ) == 0)

{ echo "Username is empty! All fields are required!"; }

If it echo's "Username is empty! All fields are required!" I want it to stop running, so that no further error messages display in my script, but if it does not echo then I continues running.

Is there a way to do this?
 

if(strlen($username ) == 0)

{ echo "Username is empty! All fields are required!";
exit();
}