The Null Coalescing Operator makes life easier for php programmers. Thanks to the php core team members and every other contributors.
Null Coalescing Operator helps you check for null value.
Previous version of php here is how it is done:
$firstname = isset($user->firstname) ? $user->firstname : 'guest';
But now cleaner and easy with PHP7:
$firstname = $user->firstname ?? 'guest';

Enjoy coding, and stay safe.