Description:
------------
---
From manual page: https://php.net/migration70
---
To whom it may concern,

I am currently migrating from php 5.6 to php 7.1.
But I found an incompatible change not listed in the document.
I would like you to include it in the document.

Kosuke Katamoto

Test script:
---------------
php5.6
```
$ php -v
PHP 5.6.38 (cli) (built: Nov 22 2018 22:26:33)
Copyright (c) 1997-2016 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies
    with Zend OPcache v7.0.6-dev, Copyright (c) 1999-2016, by Zend Technologies
    with Xdebug v2.5.5, Copyright (c) 2002-2017, by Derick Rethans
$ php -a
Interactive shell

php > $a = '';
php > $a['cat'] = 'aaa';
php > print_r($a);
Array
(
    [cat] => aaa
)
php >
```



php7.1
$ phpenv local 7.1.9
7.1.9
$ php -v
PHP 7.1.9 (cli) (built: Nov 22 2018 22:19:36) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.1.9, Copyright (c) 1999-2017, by Zend Technologies
    with Xdebug v2.6.1, Copyright (c) 2002-2018, by Derick Rethans
$ php -a
Interactive shell

php > $a = '';
php > $a['cat'] = 'aaa';
PHP Warning:  Illegal string offset 'cat' in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
php > print_r($a);
a
php >