Error while updating composer

I was trying to set up a Laravel application on ubuntu 16.04 with php 7.1. When I tried to give $composer update, I get the following error:

PHP Warning: PHP Startup: Unable to load dynamic library '/usr/lib/php/20160303/php_mbstring.dll' - /usr/lib/php/20160303/php_mbstring.dll: cannot open shared object file: No such file or directory in Unknown on line 0 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 v5.3.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.31 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.30 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.29 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.28 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.27 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.26 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.25 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.24 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.23 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.21 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.20 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.19 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.18 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.17 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.16 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.15 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.14 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.13 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.12 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.11 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - laravel/framework v5.3.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system. - Installation request for laravel/framework 5.3.* -> satisfiable by laravel/framework[v5.3.0, v5.3.1, v5.3.10, v5.3.11, v5.3.12, v5.3.13, v5.3.14, v5.3.15, v5.3.16, v5.3.17, v5.3.18, v5.3.19, v5.3.2, v5.3.20, v5.3.21, v5.3.22, v5.3.23, v5.3.24, v5.3.25, v5.3.26, v5.3.27, v5.3.28, v5.3.29, v5.3.3, v5.3.30, v5.3.31, v5.3.4, v5.3.5, v5.3.6, v5.3.7, v5.3.8, v5.3.9].

To enable extensions, verify that they are enabled in those .ini files: - /etc/php/7.1/cli/php.ini - /etc/php/7.1/cli/conf.d/10-mysqlnd.ini - /etc/php/7.1/cli/conf.d/10-opcache.ini - /etc/php/7.1/cli/conf.d/10-pdo.ini - /etc/php/7.1/cli/conf.d/15-xml.ini - /etc/php/7.1/cli/conf.d/20-calendar.ini - /etc/php/7.1/cli/conf.d/20-ctype.ini - /etc/php/7.1/cli/conf.d/20-curl.ini - /etc/php/7.1/cli/conf.d/20-dom.ini - /etc/php/7.1/cli/conf.d/20-exif.ini - /etc/php/7.1/cli/conf.d/20-fileinfo.ini - /etc/php/7.1/cli/conf.d/20-ftp.ini - /etc/php/7.1/cli/conf.d/20-gd.ini - /etc/php/7.1/cli/conf.d/20-gettext.ini - /etc/php/7.1/cli/conf.d/20-iconv.ini - /etc/php/7.1/cli/conf.d/20-json.ini - /etc/php/7.1/cli/conf.d/20-mysqli.ini - /etc/php/7.1/cli/conf.d/20-pdo_mysql.ini - /etc/php/7.1/cli/conf.d/20-phar.ini - /etc/php/7.1/cli/conf.d/20-posix.ini - /etc/php/7.1/cli/conf.d/20-readline.ini - /etc/php/7.1/cli/conf.d/20-shmop.ini - /etc/php/7.1/cli/conf.d/20-simplexml.ini - /etc/php/7.1/cli/conf.d/20-sockets.ini - /etc/php/7.1/cli/conf.d/20-sysvmsg.ini - /etc/php/7.1/cli/conf.d/20-sysvsem.ini - /etc/php/7.1/cli/conf.d/20-sysvshm.ini - /etc/php/7.1/cli/conf.d/20-tokenizer.ini - /etc/php/7.1/cli/conf.d/20-wddx.ini - /etc/php/7.1/cli/conf.d/20-xmlreader.ini - /etc/php/7.1/cli/conf.d/20-xmlwriter.ini - /etc/php/7.1/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.

What can I do to overcome this error?