Fix for mb function overload mb_substr acting different

[1emming]

When bug hunting https://github.com/fabpot/Twig/issues/1428 I ran into an issue.
I started out by changing my php.ini for the cli enviroment, I've added:
mbstring.func_overload = 2
Later I tested with adding and removing:
mbstring.internal_encoding = 'UTF-8'

It turns out mb_substr returns false and not '' when doing something like this:

$item = '';
$start = -1;
$length = 1;
$charset = 'UTF-8';
$a = mb_substr($item, $start, $length, $charset);
var_dump($a);
die;

I'm not sure this will fix 1428 since I don't have enough information for this, but this PR should at least fix one issue.

[fabpot]

Is it possible to add a test that covers this issue?

[1emming]

When setting the option in /test/bootstrap.php:

ini_set('mbstring.func_overload', '2');

all goes well. When setting it directly in the php.ini it goes wrong (the Intergration tests for first and last fail). This is really and issue with mbstring, but changes to that are slow :/

(While odd this also seems the case for some users running Windows & Apache2 who try to set it the .htaccess (doesn't work either @see http://php.net/manual/en/mbstring.overload.php).)

Therefore I cannot provide a test since it only happends when setting it in the php.ini file. I've tried it on Ubuntu and Debian and in both cases it goes wrong. Could you try running the test (on master) with the change in your php.ini ?

[fabpot]

What about just casting to a string? return (string) mb_substr(...);

[1emming]

@fabpot I agree on the cast over the result check, please last commit :)

[fabpot]

A small thing: you inadvertently changed the file rights to 0755, can you revert this change?

[fabpot]

Thank you @1emming.