求助下各位大牛，laravel用composer安装图片处理出现这个问题

E:\WWW\laravel>composer require intervention/image
You are running Composer with SSL/TLS protection disabled.
Warning: Accessing packagist.phpcomposer.com over http which is an insecure protocol.
Using version ^2.3 for intervention/image
./composer.json has been updated

php artisan clear-compiled
Loading composer repositories with package information
Warning: Accessing packagist.phpcomposer.com over http which is an insecure protocol.
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Problem 1
- laravel/framework v5.2.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.22 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- Installation request for laravel/framework (locked at v5.2.22, required as 5.2.*) -> satisfiable by laravel/framework[v5.2.22].

Installation failed, reverting ./composer.json to its original content.

这是什么问题？求大佬解答一下，谢谢