Bonjour  j'ai un soucis avec l'installation de laravel. je voulais faire un nouveau projet laravel. j'ai ubuntu 16.04 LTS. j'ai installer LAMPP, puis composer et maintenant je voulais installer laravel pour debuter un projet. je me suis rendu dans le dossier HTDOCS de lampp puis fais un:

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

l'installation a débuté pour finalement s'arreter sur ces erreurs.

Installing laravel/laravel (v5.4.23)
  - Installing laravel/laravel (v5.4.23)
    Loading from cache

Created project in site
> 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 v5.4.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.23 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.21 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.20 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.19 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.18 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.17 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.16 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.15 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.14 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.13 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.12 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.11 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - Installation request for laravel/framework 5.4.* -> satisfiable by laravel/framework[v5.4.0, v5.4.1, v5.4.10, v5.4.11, v5.4.12, v5.4.13, v5.4.14, v5.4.15, v5.4.16, v5.4.17, v5.4.18, v5.4.19, v5.4.2, v5.4.20, v5.4.21, v5.4.22, v5.4.23, v5.4.3, v5.4.4, v5.4.5, v5.4.6, v5.4.7, v5.4.8, v5.4.9].

  To enable extensions, verify that they are enabled in those .ini files:
    - /etc/php/7.0/cli/php.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/20-calendar.ini
    - /etc/php/7.0/cli/conf.d/20-ctype.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-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-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
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.
root@beevisible:/opt/lampp/htdocs# 

je suis apres rentrer dans le dossier site puis un coup de

composer update 

et voici les erreurs.

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 v5.4.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.23 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.21 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.20 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.19 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.18 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.17 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.16 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.15 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.14 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.13 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.12 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.11 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - laravel/framework v5.4.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
    - Installation request for laravel/framework 5.4.* -> satisfiable by laravel/framework[v5.4.0, v5.4.1, v5.4.10, v5.4.11, v5.4.12, v5.4.13, v5.4.14, v5.4.15, v5.4.16, v5.4.17, v5.4.18, v5.4.19, v5.4.2, v5.4.20, v5.4.21, v5.4.22, v5.4.23, v5.4.3, v5.4.4, v5.4.5, v5.4.6, v5.4.7, v5.4.8, v5.4.9].

  To enable extensions, verify that they are enabled in those .ini files:
    - /etc/php/7.0/cli/php.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/20-calendar.ini
    - /etc/php/7.0/cli/conf.d/20-ctype.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-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-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
  You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

L'erreur est pourtant explicite: il manque l'extension mbstring.

Go apt install php-mbstring.