Per RFC: https://wiki.php.net/rfc/short_ternary_equal_operator

The following two expressions become effectively identical:

$x = $x ?: $y;
$x ?:= $y;

Slightly less efficient, but more reliable:
  * Try a RW fetch to on LHS. This will build out the container tree on the way to checking its value
  * If the var is truthy, jump ahead and everybody's happy.
  * If the var is falsy, evaluate the default expression and refetch the storage as a W fetch for assignment.