Installation keeps complaining that Unicode library (mbstring) should be disabled, even though it seems it already is I'm trying to install Drupal 8.1.3.
(Apache 2.4.7, PHP 5.5.37, Windows 7)

In the "Verify Requirements" step Drupal keeps complaining


  Unicode library -- Error -- Multibyte string input conversion in PHP
  is active and must be disabled. Check the php.ini mbstring.http_input
  setting. Please refer to the PHP mbstring documentation for more
  information.


However, I already have the following in my php.ini

mbstring.http_input = pass


and the diagnostic output produced by a call to phpinfo(); contains

zend.multibyte  | Off | Off


So I am not sure whether


I was unsuccessful in switching off the multibyte string input conversion,
I am looking at the wrong configuration files / settings, or
Something is wrong with Drupal's detection of these setting (see for instance this discussion on a failing detection).
http://php.net/manual/en/mbstring.configuration.php#ini.mbstring.http-input

Drupal's code is checking it in core/lib/Drupal/Component/Utility/Unicode.php, but it won't even check if unless PHP version is &lt; 5.6.0.  If you are using PHP 5.5.x here's the check:

  if (ini_get('mbstring.http_output') != 'pass') {
    static::$status = static::STATUS_ERROR;
    return 'mbstring.http_output';
  } 


So I would double check the phpinfo via the Drupal interface at /admin/reports/status/php to make sure you are changing the config file that's used by PHP for Drupal.