问题1
Installation request for laravel/framework v5.2.16 -> satisfiable by laravel/framework[v5.2.16].
laravel/framework v5.2.16 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
要启用扩展程序，请验证它们是否已在.ini个文件中启用：
/etc/php/7.0/cli/php.ini
/etc/php/7.0/cli/conf.d/10mysqlnd.ini
/etc/php/7.0/cli/conf.d/10opcache.ini
/etc/php/7.0/cli/conf.d/10pdo.ini
/etc/php/7.0/cli/conf.d/20calendar.ini
/etc/php/7.0/cli/conf.d/20ctype.ini
/etc/php/7.0/cli/conf.d/20exif.ini
/etc/php/7.0/cli/conf.d/20fileinfo.ini
/etc/php/7.0/cli/conf.d/20ftp.ini
/etc/php/7.0/cli/conf.d/20gettext.ini
/etc/php/7.0/cli/conf.d/20iconv.ini
/etc/php/7.0/cli/conf.d/20json.ini
/etc/php/7.0/cli/conf.d/20mysqli.ini
/etc/php/7.0/cli/conf.d/20pdo_mysql.ini
/etc/php/7.0/cli/conf.d/20phar.ini
/etc/php/7.0/cli/conf.d/20posix.ini
/etc/php/7.0/cli/conf.d/20readline.ini
/etc/php/7.0/cli/conf.d/20shmop.ini
/etc/php/7.0/cli/conf.d/20sockets.ini
/etc/php/7.0/cli/conf.d/20sysvmsg.ini
/etc/php/7.0/cli/conf.d/20sysvsem.ini
/etc/php/7.0/cli/conf.d/20sysvshm.ini
/etc/php/7.0/cli/conf.d/20tokenizer.ini
/etc/php/7.0/cli/conf.d/20zlib.ini
您也可以在终端内运行php --ini以查看PHP在CLI模式下使用的文件。