Laravel 5：在ubuntu中创建laravel项目时出错

我在Windows 7中创建了一个laravel项目，它运行正常 . 但是当我在ubuntu中通过命令提示符使用命令创建它时：composer create-project laravel / laravel lara，得到错误

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

Problem 1
- laravel/framework v5.5.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.28 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.27 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.26 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.25 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.24 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.23 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.21 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.20 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.19 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.18 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.17 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.16 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.15 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.14 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.13 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.12 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.11 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.5.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- Installation request for laravel/framework 5.5.* -> satisfiable by laravel/framework[v5.5.0, v5.5.1, v5.5.10, v5.5.11, v5.5.12, v5.5.13, v5.5.14, v5.5.15, v5.5.16, v5.5.17, v5.5.18, v5.5.19, v5.5.2, v5.5.20, v5.5.21, v5.5.22, v5.5.23, v5.5.24, v5.5.25, v5.5.26, v5.5.27, v5.5.28, v5.5.3, v5.5.4, v5.5.5, v5.5.6, v5.5.7, v5.5.8, v5.5.9].

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/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-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.

像这样

但它正在opt / lampp / htdocs中创建项目文件夹，而我在浏览器中运行此错误，如下所示：

Warning: require(/opt/lampp/htdocs/l5/public/../vendor/autoload.php): failed to open stream: No such file or directory in /opt/lampp/htdocs/l5/public/index.php on line 24

Fatal error: require(): Failed opening required '/opt/lampp/htdocs/l5/public/../vendor/autoload.php' (include_path='.:/opt/lampp/lib/php') in /opt/lampp/htdocs/l5/public/index.php on line 24

请帮我解决这个问题，我是laravel和ubuntu的初学者 . 任何帮助将不胜感激 .

提前致谢 .