How to install and issue laravel 5.6 in ubuntu 16.04?

I upgarde PHP 7.0 to 7.1 with this command:

1. sudo add-apt-repository ppa:ondrej/php
2. sudo apt-get update
3. sudo apt-get install php7.1

and run this command from laravel.com:

composer create-project --prefer-dist laravel/laravel blog

After run this code, show error:

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

Problem 1
- laravel/framework v5.6.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework 5.6.x-dev requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- Installation request for laravel/framework 5.6.* -> satisfiable by laravel/framework[5.6.x-dev, v5.6.0].

To enable extensions, verify that they are enabled in those .ini files:
- /etc/php/7.1/cli/php.ini
- /etc/php/7.1/cli/conf.d/10-opcache.ini
- /etc/php/7.1/cli/conf.d/10-pdo.ini
- /etc/php/7.1/cli/conf.d/20-calendar.ini
- /etc/php/7.1/cli/conf.d/20-ctype.ini
- /etc/php/7.1/cli/conf.d/20-exif.ini
- /etc/php/7.1/cli/conf.d/20-fileinfo.ini
- /etc/php/7.1/cli/conf.d/20-ftp.ini
- /etc/php/7.1/cli/conf.d/20-gettext.ini
- /etc/php/7.1/cli/conf.d/20-iconv.ini
- /etc/php/7.1/cli/conf.d/20-json.ini
- /etc/php/7.1/cli/conf.d/20-phar.ini
- /etc/php/7.1/cli/conf.d/20-posix.ini
- /etc/php/7.1/cli/conf.d/20-readline.ini
- /etc/php/7.1/cli/conf.d/20-shmop.ini
- /etc/php/7.1/cli/conf.d/20-sockets.ini
- /etc/php/7.1/cli/conf.d/20-sysvmsg.ini
- /etc/php/7.1/cli/conf.d/20-sysvsem.ini
- /etc/php/7.1/cli/conf.d/20-sysvshm.ini
- /etc/php/7.1/cli/conf.d/20-tokenizer.ini
You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

How to issue this problem?

via Chebli Mohamed