Windowsでのcomposer install/updateエラー（requires ext-mbstring）

composer installで以下のエラーが出る場合。

 Problem 1
    - Installation request for maknz/slack 1.7.0 -> satisfiable by maknz/slack[1.7.0].
    - maknz/slack 1.7.0 requires ext-mbstring * -> the requested PHP extension mbstring is missing from your system.

php.iniに以下を追記で対処。

extension=php_mbstring.dll