Description:
------------
When certain strings are unserialized they do not reserialize properly.

Reproduce code:
---------------
<?php
$x = array();
$x[] = &$x;

var_dump($x);
$z = serialize($x);
echo $z . "\n";
var_dump(unserialize($z));

echo "\n\n\n";

$x = unserialize("a:1:{i:0;R:1;}");
var_dump($x);
$z = serialize($x);
echo $z . "\n\n";
var_dump(unserialize($z));


Expected result:
----------------
array(1) {
  [0]=>
  &array(1) {
    [0]=>
    &array(1) {
      [0]=>
      *RECURSION*
    }
  }
}
a:1:{i:0;a:1:{i:0;R:2;}}
array(1) {
  [0]=>
  &array(1) {
    [0]=>
    &array(1) {
      [0]=>
      *RECURSION*
    }
  }
}



array(1) {
  [0]=>
  &array(1) {
    [0]=>
    &array(1) {
      [0]=>
      *RECURSION*
    }
  }
}
a:1:{i:0;R:1;}

array(1) {
  [0]=>
  &array(1) {
    [0]=>
    &array(1) {
      [0]=>
      *RECURSION*
    }
  }
}


Actual result:
--------------
array(1) {
  [0]=>
  &array(1) {
    [0]=>
    &array(1) {
      [0]=>
      *RECURSION*
    }
  }
}
a:1:{i:0;a:1:{i:0;R:2;}}
array(1) {
  [0]=>
  &array(1) {
    [0]=>
    &array(1) {
      [0]=>
      *RECURSION*
    }
  }
}



array(1) {
  [0]=>
  array(1) {
    [0]=>
    array(1) {
      [0]=>
      *RECURSION*
    }
  }
}
a:1:{i:0;a:1:{i:0;a:1:{i:0;N;}}}

array(1) {
  [0]=>
  array(1) {
    [0]=>
    array(1) {
      [0]=>
      NULL
    }
  }
}