composerをインストールしようとするとエラーでます。どのように解消すればよろしいでしょうか？

Question

composerをインストールしようと以下のようなエラーでます。どのように解消すればよろしいでしょうか？
ちなみにlaravelを使う際に使います。

C:\repository\master>composer install
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 v5.2.9 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.8 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.7 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.6 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.5 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.4 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.39 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.38 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.37 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.36 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.35 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.34 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.33 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.32 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.31 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.30 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.3 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.29 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.28 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.27 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.26 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.25 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.24 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.23 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 v5.2.21 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.20 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.2 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.19 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.18 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.17 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.16 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.15 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.14 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.13 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.12 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.11 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.10 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.1 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- laravel/framework v5.2.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.
- Installation request for laravel/framework 5.2.* -> satisfiable by laravel/framework[v5.2.0, v5.2.1, v5.2.10, v5.2.11, v5.2.12, v5.2.13, v5.2.14, v5.2.15, v5.2.16, v5.2.17, v5.2.18, v5.2.19, v5.2.2, v5.2.20, v5.2.21, v5.2.22, v5.2.23, v5.2.24, v5.2.25, v5.2.26, v5.2.27, v5.2.28, v5.2.29, v5.2.3, v5.2.30, v5.2.31, v5.2.32, v5.2.33, v5.2.34, v5.2.35, v5.2.36, v5.2.37, v5.2.38, v5.2.39, v5.2.4, v5.2.5, v5.2.6, v5.2.7, v5.2.8, v5.2.9].

To enable extensions, verify that they are enabled in those .ini files:
- C:\php\php.ini
You can also run php --ini inside terminal to see which files are used by PHP in CLI mode.

Answer 1

Problem 1は composer.lockの内容がcomposer.jsonの最新の内容に追従していないので、
先にcomposer updateしてねという意味合いだと思います。

Problem 2はlaravel/frameworkがPHPエクステンションのmbstringを必須としているけど、
システムで有効になっていないという内容でしょう。同エクステンションは初期状態では
無効になっているはずなので、php.ini内のコメントアウトされている部分を外してから
apacheを再起動してみてください。

//php.ini
//セミコロンを外す
;extension=php_mbstring.dll

2を解消してから1をしたほうがいいと思います。もしかしたら他にも拡張の有効化や
インストールを要求されるかもしれませんが、都度解決してあげれば大丈夫です。