Description:
------------
---
From manual page: http://www.php.net/function.mb-substr
---
"Return Values" says this returns a string, which is true for most cases but not true for at least one edge case: `var_dump(mb_substr(array(), 0, 1));` returns NULL in PHP 5.3+ (but returns "A" before PHP 5.3). 

I haven't checked the PHP source, I guess these it can be determined when exactly does mb_substr return NULL.

Test results with https://3v4l.org/CuDLX:

Output for 5.3.0 - 5.4.45, 5.5.0 - 5.5.34, 5.5.35 - 5.6.26, hhvm-3.10.0 - 3.14.4, 7.0.0 - 7.1.0RC2
`
Warning: mb_substr() expects parameter 1 to be string, array given in /in/CuDLX on line 3
NULL
`

Output for 4.3.0 - 5.2.17
`
Notice: Array to string conversion in /in/CuDLX on line 3
string(1) "A"
`

Test script:
---------------
var_dump(mb_substr(array(), 0, 1));

Expected result:
----------------
The documentation should define when does mb_substr returns NULL.

Actual result:
--------------
The documentation suggests mb_substr always returns a string.