Your expectation is wrong:

In your second example two variables refer to the same NULL value. (first element of array and variable $a)

Compare the output of the following two scripts

<?php
$x = array(&$a);
var_dump($x);
?>
Actual result:
--------------
array(1) {
  [0]=>
  &NULL
}

<?php
$x = array(&$a);
unset($a);
var_dump($x);
?>
Actual result:
--------------
array(1) {
  [0]=>
  NULL
}

In the third example you assign the array from second example to $a, but $a is a reference, so the assignment will update the first element of array too.

<?php
$b = array(&$a);
var_dump($b);
$a = $b;
var_dump($b);
?>
Actual result:
--------------
array(1) {
  [0]=>
  &NULL
}
array(1) {
  [0]=>
  &array(1) {
    [0]=>
    &array(1) {
      [0]=>
      *RECURSION*
    }
  }
}


All results are the same as they should be.