Adding new reference to any item in array affects behaviour of the array

I've seen many questions about this (one of them here)

$a = array('a', 'b', 'c', 'd');

foreach ($a as &$v) { }
foreach ($a as $v) { }

print_r($a);

and I get the answers, you shouldn't be recycling the $v variable name. But..!

Can someone explain to me why this code

$a = array('a', 'b', 'c', 'd');

foreach ($a as &$v) { }

var_dump($a);

prints this (note that the last element is a reference)?

array(4) {
  [0]=>
  string(1) "a"
  [1]=>
  string(1) "b"
  [2]=>
  string(1) "c"
  [3]=>
  &string(1) "d"
}

I expect it to be just a value, only $v should be a reference. In other words, I'm dealing with the same behaviour as described (and well documented) in the quoted question, although I'm not reusing $v variable incorrectly.