Description:
------------
Calling json_encode() with an instance of a class that implements the 
JsonSerializable has a reference to itself (recursion), PHP coredumps (CLI) or 
exits with 'Allowed memory size of ... bytes exhusted'.

However, recursion in an instance of a class that does not implement the 
JsonSerializable interface is detected, a warning is shown and the actual JSON 
representation of the recursive reference is null.

In some situations, it tried to continously alocate memory, and eventually failed 
with the 'Allowed memory size of XXXX bytes exhausted' message.

Test script:
---------------
<?php // file: jsonserialize.php

ini_set('display_errors', 'on');
error_reporting(-1);
set_time_limit(1);
ini_set('memory_limit','512M');

class JsonTest1 {
    public $test;
    public $me;
    public function __construct() {
        $this->test = '123';
        $this->me  = $this;
    }
}

class JsonTest2 implements JsonSerializable {
    public $test;
    public function __construct() {
        $this->test = '123';
    }
    public function jsonSerialize() {
        return array(
            'test' => $this->test,
            'me'   => $this
        );
        // same results with:
        // return get_object_vars($this);
    }
}


$obj1 = new JsonTest1();
echo json_encode($obj1);

echo "\n==\n";

$obj2 = new JsonTest2();
echo json_encode($obj2);


Expected result:
----------------
Warning: json_encode(): recursion detected in jsonserialize.php on line 34
{"test":"123","me":{"test":"123","me":null}}
==
Warning: json_encode(): recursion detected in jsonserialize.php on line 39
{"test":"123","me":{"test":"123","me":null}}

Actual result:
--------------
Warning: json_encode(): recursion detected in /var/www/jsonserialize.php on line 
34
{"test":"123","me":{"test":"123","me":null}}
==
Segmentation fault (core dumped)