Description:
------------
---
From manual page: http://www.php.net/function.mb-regex-set-options
---

The documentation says that mb_regex_set_options should return the previous results, 

> The previous options. If options is omitted, it returns the string that describes the current options.


This is quite useful so you can change the options and easily set it back to the defaults after you are done with mb_regex. However, it seems to do something totally different, returning the current options instead.

It seems like the internal function is designed for it:

https://github.com/php/php-src/blob/a5e80b22e11c2db7fd5ff07b5b7a28f80745e89b/ext/mbstring/php_mbregex.c#L1624

however this functionality is not used in the wrapping function:

https://github.com/php/php-src/blob/a5e80b22e11c2db7fd5ff07b5b7a28f80745e89b/ext/mbstring/php_mbregex.c#L1660

null is passed in.

Test script:
---------------
```
php > var_dump(mb_regex_set_options("m"));
string(2) "mr"
php > var_dump(mb_regex_set_options("mdi"));
string(3) "imd"
php > var_dump(mb_regex_set_options("m"));
string(2) "mr"
php > var_dump(mb_regex_set_options("a"));
string(1) "r"
```