Most efficient way to shorten an if statement

Question

How would one shorten an if statement if code is as below ?

$a = null;
$b = "foo";

if ((empty($a) && !empty($b)) || (!empty($a) && empty($b)) {}

Answer 1

The test seems to be checking that only 1 field contains a value, so you could check

$a = null;
$b = "foo";

if (empty($a) != empty($b)) {}

Answer 2

$a = null;
$b = "foo";
if(empty($a) xor empty($b)){}