?? – it asks “does this variable is set and not null”
It returns its first operand if it exists and is not null; otherwise it returns its second operand.
$lng = $lng ?? '';
Or, if you are dealing with integers: at first it checks does this expression in parentheses is true – exists and is not null, and then returns same value, but as integer value
$lng = (int)($lng ?? 0);
Thanks to: https://www.php.net/manual/en/migration70.new-features.php