Apparent evidence PHP json_encode not returning FALSE on error

gistfile1.txt

    /**
     * Evidence that PHP json_encode is not returning FALSE on error.
     * (Reference: http://us1.php.net/json_encode)
     * 
     * Env:
     * 
     * PHP 5.3.10-1ubuntu3.9 with Suhosin-Patch (cli) (built: Dec 12 2013 04:27:25) 
     *   Copyright (c) 1997-2012 The PHP Group
     *   Zend Engine v2.3.0, Copyright (c) 1998-2012 Zend Technologies
     *   with Xdebug v2.2.3, Copyright (c) 2002-2013, by Derick Rethans
     */
    public function testApparentPHPJsonEncodeBug()
    {
        
        // json_encode cannot handle this input. It usually causes this error:
        // "json_encode(): Invalid UTF-8 sequence in argument" to appear in 
        // error logs. I will instead silence the error in order to observe 
        // the result value.
        $result = @json_encode(pack("H*" ,'c32e'));

        // Unfortunately I am getting a string back.
        $this->assertTrue(is_string($result));
        
        // The contents of the string are "null"
        $this->assertEquals("null", $result);
        
    }