Amazon LinuxでAWSチュートリアルに沿って、LAMPを環境後、composerをインストールしましたが、cakeのプロジェクトを作成して、以下のエラーのようなものがでてきました。
ブラウザで確認しても、真っ白な状態になっています。
何かCakephp3のチュートリアルにのっていないパッケージが必要になりますか？

composer create-project --prefer-dist cakephp/app cake3
Installing cakephp/app (3.2.0)
- Installing cakephp/app (3.2.0)
Loading from cache

Created project in cake3
Loading composer repositories with package information
Installing dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Problem 1
- cakephp/cakephp 3.2.x-dev requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- cakephp/cakephp 3.2.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- cakephp/cakephp 3.2.0-RC1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- cakephp/cakephp 3.2.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- Installation request for cakephp/cakephp ~3.2 -> satisfiable by cakephp/cakephp[3.2.0, 3.2.0-RC1, 3.2.1, 3.2.x-dev].

To enable extensions, verify that they are enabled in those .ini files:
- /etc/php.ini
- /etc/php-5.6.d/20-bz2.ini
- /etc/php-5.6.d/20-calendar.ini
- /etc/php-5.6.d/20-ctype.ini
- /etc/php-5.6.d/20-curl.ini
- /etc/php-5.6.d/20-dom.ini
- /etc/php-5.6.d/20-exif.ini
- /etc/php-5.6.d/20-fileinfo.ini
- /etc/php-5.6.d/20-ftp.ini
- /etc/php-5.6.d/20-gettext.ini
- /etc/php-5.6.d/20-iconv.ini
- /etc/php-5.6.d/20-intl.ini
- /etc/php-5.6.d/20-mysqlnd.ini
- /etc/php-5.6.d/20-pdo.ini
- /etc/php-5.6.d/20-phar.ini
- /etc/php-5.6.d/20-posix.ini
- /etc/php-5.6.d/20-shmop.ini
- /etc/php-5.6.d/20-simplexml.ini
- /etc/php-5.6.d/20-sockets.ini
- /etc/php-5.6.d/20-sqlite3.ini
- /etc/php-5.6.d/20-sysvmsg.ini
- /etc/php-5.6.d/20-sysvsem.ini
- /etc/php-5.6.d/20-sysvshm.ini
- /etc/php-5.6.d/20-tokenizer.ini
- /etc/php-5.6.d/20-xml.ini
- /etc/php-5.6.d/20-xmlwriter.ini
- /etc/php-5.6.d/20-xsl.ini
- /etc/php-5.6.d/20-zip.ini
- /etc/php-5.6.d/30-mysql.ini
- /etc/php-5.6.d/30-mysqli.ini
- /etc/php-5.6.d/30-pdo_mysql.ini
- /etc/php-5.6.d/30-pdo_sqlite.ini
- /etc/php-5.6.d/30-wddx.ini
- /etc/php-5.6.d/30-xmlreader.ini
- /etc/php-5.6.d/40-json.ini
- /etc/php-5.6.d/php.ini
You can also run `php --ini` inside terminal to see which files are used by PHP in CLI mode.

bin/cake serverと打つと、

PHP Warning:  require(/var/www/html/cake3/vendor/autoload.php): failed to open stream: No such file or directory in /var/www/html/cake3/config/bootstrap.php on line 23
PHP Fatal error:  require(): Failed opening required '/var/www/html/cake3/vendor/autoload.php' (include_path='.:/usr/share/pear:/usr/share/php') in /var/www/html/cake3/config/bootstrap.php on line 23

とエラーがでます。