Description:
------------
Exception::__toString() method will cause circular references.
And therefore breaks some *array* related functions.

Version 7.0.11 is fine.

Tested on CentOS 6/7 x64, packages from
 http://rpms.famillecollet.com/enterprise/{6,7}/php70/x86_64/repoview/

Tested on openSUSE Leap 42.1 x64, packages from
 http://download.opensuse.org/repositories/devel:/languages:/php/openSUSE_Leap_42.1/

Test script:
---------------
<?php
$e = new Exception();

// This line cause problem :(
// Comment it to see the difference.
(string) $e;

// This line show the clue (PHP Warning: ...).
var_export($e);

// This line will be affected.
var_export(json_encode([$e]));

// But This line still OK.
var_export(json_encode($e));


Expected result:
----------------
# var_export($e)
Exception::__set_state(array(
   'message' => '',
   'string' => '',
   'code' => 0,
   'file' => 'demo.php',
   'line' => 2,
   'trace' => 
  array (
  ),
   'previous' => NULL,
))

# var_export(json_encode([$e]))
'[{}]'

# var_export(json_encode($e))
'{}'


Actual result:
--------------
# var_export($e)
PHP Warning:  var_export does not handle circular references in demo.php on line 9
NULL

# var_export(json_encode([$e]))
false

# var_export(json_encode($e))
'{}'