Fixes #6697: Added `yii\helpers\Url::current()` method that allows adding or removing parameters from current URL

always use getConnection()

ensure PDO instance has been created at this point

always use unique directory for each test

http://en.wikipedia.org/wiki/Year_2038_problem

formatting date and time values for years >=2038 or <=1901 is now possible also with intl extension is
installed. 32bit systems will fall back to the PHP implementation because intl uses a 32bit UNIX timestamp internally.
On a 64bit system the intl formatter is used in all cases if installed.

fixes #5577

fixes #6748

Fixes #6398: Added support for specifying dependent component in terms of a configuration array for classes such as `DbCache`

fixes #6717

Fixes #6632: `yii\di\Container::get()` did not handle config parameter correctly when it is passed as a constructor parameter

Fixes #5494: Added support for specifying a menu header as a configuration array in `yii\bootstrap\Dropdown`

fixes #5503

fixes #6263 and has no problem with PHP Bug:

> it is a PHP bug: https://bugs.php.net/bug.php?id=45543
> Fixed in this commit: php/php-src@22dba2f#diff-7b738accc3d60f74c259da18588ddc5dL2996
>
> Fixed in PHP >5.4.26 and >5.5.10. http://3v4l.org/mlZX7

Fixes #5627: Added `yii cache/flush-schema` console command to flush DB schema cache of a given database connection

fixes #5448

fixes #6164

Fixes #4823 and #6005: `yii message` accuracy and error handling were improved by using PHP tokenizer instead of regular expressions. Removed eval() as well.

fixes #5683

Fixes #4643: Extra options specified in `yii\widgets\Breadcrumbs::links` will be treated as HTML attributes for the generated hyperlinks