yii2 ошибка при установке. Your requirements could not be resolved to an installable set of packages.

Пытаюсь установить yii2 с помощью команды:

composer create-project --prefer-dist yiisoft/yii2-app-basic basic

Ответ:

Installing yiisoft/yii2-app-basic (2.0.12)
  - Installing yiisoft/yii2-app-basic (2.0.12): Downloading (100%)         
Created project in basic
Loading composer repositories with package information
Updating dependencies (including require-dev)

А затем ошибка:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - yiisoft/yii2 2.0.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.12 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.11.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.11.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.11 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - yiisoft/yii2 2.0.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - Installation request for yiisoft/yii2 ~2.0.5 -> satisfiable by yiisoft/yii2[2.0.10, 2.0.11, 2.0.11.1, 2.0.11.2, 2.0.12, 2.0.5, 2.0.6, 2.0.7, 2.0.8, 2.0.9].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php/7.0/cli/php.ini
    - /etc/php/7.0/cli/conf.d/10-mysqlnd.ini
    - /etc/php/7.0/cli/conf.d/10-opcache.ini
    - /etc/php/7.0/cli/conf.d/10-pdo.ini
    - /etc/php/7.0/cli/conf.d/15-xml.ini
    - /etc/php/7.0/cli/conf.d/20-calendar.ini
    - /etc/php/7.0/cli/conf.d/20-ctype.ini
    - /etc/php/7.0/cli/conf.d/20-dom.ini
    - /etc/php/7.0/cli/conf.d/20-exif.ini
    - /etc/php/7.0/cli/conf.d/20-fileinfo.ini
    - /etc/php/7.0/cli/conf.d/20-ftp.ini
    - /etc/php/7.0/cli/conf.d/20-gettext.ini
    - /etc/php/7.0/cli/conf.d/20-iconv.ini
    - /etc/php/7.0/cli/conf.d/20-json.ini
    - /etc/php/7.0/cli/conf.d/20-mysqli.ini
    - /etc/php/7.0/cli/conf.d/20-pdo_mysql.ini
    - /etc/php/7.0/cli/conf.d/20-phar.ini
    - /etc/php/7.0/cli/conf.d/20-posix.ini
    - /etc/php/7.0/cli/conf.d/20-readline.ini
    - /etc/php/7.0/cli/conf.d/20-shmop.ini
    - /etc/php/7.0/cli/conf.d/20-simplexml.ini
    - /etc/php/7.0/cli/conf.d/20-sockets.ini
    - /etc/php/7.0/cli/conf.d/20-sysvmsg.ini
    - /etc/php/7.0/cli/conf.d/20-sysvsem.ini
    - /etc/php/7.0/cli/conf.d/20-sysvshm.ini
    - /etc/php/7.0/cli/conf.d/20-tokenizer.ini
    - /etc/php/7.0/cli/conf.d/20-wddx.ini
    - /etc/php/7.0/cli/conf.d/20-xmlreader.ini
    - /etc/php/7.0/cli/conf.d/20-xmlwriter.ini
    - /etc/php/7.0/cli/conf.d/20-xsl.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

Подскажите, пожалуйста, что это означает и как исправить.