ComposerでLaravelをインストールできない！

#問題点
Composerを使ってLaravelをインストールしようとすると、[InvalidArgumentException]が吐き出されて、インストールできない。

Composer (version 1.7.2) successfully installed to: /home/vagrant/laravel_lessons/composer.phar
Use it: php composer.phar

[vagrant@localhost laravel_lessons]$ php -r "unlink('composer-setup.php');"
[vagrant@localhost laravel_lessons]$ php composer.phar create-project --prefer-dist laravel/laravel myblog
Installing laravel/laravel (v5.7.0)


  [InvalidArgumentException]
  Project directory myblog/ is not empty.


create-project [-s|--stability STABILITY] [--prefer-source] [--prefer-dist] [--repository REPOSITORY] [--repository-url REPOSITORY-URL] [--dev] [--no-dev] [--no-custom-installers] [--no-scripts] [--no-progress] [--no-secure-http] [--keep-vcs] [--remove-vcs] [--no-install] [--ignore-platform-reqs] [--] [<package>] [<directory>] [<version>]

【追記】
myblogを空にして再びlaravelをインストールしようと思ったら、違うエラーコードが出てきた。

[vagrant@localhost laravel_lessons]$ php composer.phar create-project --prefer-dist laravel/laravel myblog
Installing laravel/laravel (v5.7.0)
    Failed to download laravel/laravel from dist: The zip extension and unzip command are both missing, skipping.
Your command-line PHP is using multiple ini files. Run `php --ini` to show them.
    Now trying to download from source
  - Installing laravel/laravel (v5.7.0): Cloning b0651d2467
Created project in myblog
> @php -r "file_exists('.env') || copy('.env.example', '.env');"
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - laravel/framework v5.7.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.7.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.7.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.7.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.7.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.7.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework 5.7.x-dev requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - Installation request for laravel/framework 5.7.* -> satisfiable by laravel/framework[5.7.x-dev, v5.7.0, v5.7.1, v5.7.2, v5.7.3, v5.7.4, v5.7.5].

  To enable extensions, verify that they are enabled in your .ini files:
    - /etc/php.ini
    - /etc/php.d/20-bz2.ini
    - /etc/php.d/20-calendar.ini
    - /etc/php.d/20-ctype.ini
    - /etc/php.d/20-curl.ini
    - /etc/php.d/20-exif.ini
    - /etc/php.d/20-fileinfo.ini
    - /etc/php.d/20-ftp.ini
    - /etc/php.d/20-gettext.ini
    - /etc/php.d/20-iconv.ini
    - /etc/php.d/20-json.ini
    - /etc/php.d/20-phar.ini
    - /etc/php.d/20-sockets.ini
    - /etc/php.d/20-tokenizer.ini
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

どうやったらLaravelをインストールできるか教えてください