public function test(AAA(BBB) ?$value)                                              {}

AAA type hints an interface, BBB is the predefined autobox class which must implement AAA

// -----------------------------------------------------------------------------------------

interface iMyString{}
class MyString implements iMyString{}
class MyUTF8String implements iMyString{}

class BinThingy {
    public function sendData (iMyString(MyString) ?$value)                          {}
}

class UTF8Thingy extends BinThingy {
    /** @override */
    public function sendData (iMyString(MyUTF8String) ?$value)                      {}
    // ^ works, signature is the same, only default autobox class changes
}

(new BinThingy)->sendData("foo");  // creates a MyString      w/o calling the ctor and fills it via MyString->__autobox("foo")
(new UTF8Thingy)->sendData("foo"); // creates a MyUTF8String  w/o calling the ctor and fills it via MyUTF8String->__autobox("foo")

// all valid:
(new BinThingy)->sendData(new MyString("foo"));
(new BinThingy)->sendData(new MyUTF8String("foo"));
(new UTF8Thingy)->sendData(new MyString("foo"));
(new UTF8Thingy)->sendData(new MyUTF8String("foo"));

// -----------------------------------------------------------------------------------------

class MyString implements iMyString
{
    function __autobox($value)
    {
         if(is_string($value))
                 $this->storage = $value;
         else ....
    }
}

class MyUTF8String implements iMyString
{
    function __autobox($value)
    {
         if(is_string($value))
                 $this->storage = $value;
         else ....
    }
}