Nullable
Nullable is the name of the NULL value, when used as a type. Until PHP 8.0, it was marked as a question mark with types, and since PHP 8.0, it is also marked with its own name, as a union type.
<?php
// argument has the short nullable type
// return type has the long form nullable type
function foo(?A $a) : null | B {
// code
}
?>