The example might have been using a constant. Those are pre-defined values like
define('USERNAME', '
[email protected]');
// or
const USERNAME = '
[email protected]';
In the first one, the constant is created with a function, and the name of the constant needs to be a string because it's passed as a value to the define() function.
The second one shows a special syntax for creating constants, where the name of the constant doesn't have to be a string because that's how the syntax works: name of constant = value of constant.
Note that for both of those syntaxes, the email value is still a string.