window yii2 安装插件 报yiisoft/yii2 2.0.x-dev requires ext-mbstring错

 Problem 1
        - yiisoft/yii2 2.0.x-dev requires ext-mbstring * -> the requested PHP extens
    ion mbstring is missing from your system.
        - yiisoft/yii2 2.0.9 requires ext-mbstring * -> the requested PHP extension
    mbstring is missing from your system.
        - yiisoft/yii2 2.0.8 requires ext-mbstring * -> the requested PHP extension
    mbstring is missing from your system.
        - yiisoft/yii2 2.0.7 requires ext-mbstring * -> the requested PHP extension
    mbstring is missing from your system.
        - yiisoft/yii2 2.0.6 requires ext-mbstring * -> the requested PHP extension
    mbstring is missing from your system.
        - Installation request for yiisoft/yii2 ~2.0.6 -> satisfiable by yiisoft/yii
    2[2.0.6, 2.0.7, 2.0.8, 2.0.9, 2.0.x-dev].

      To enable extensions, verify that they are enabled in those .ini files:
        - I:\android_developer\httpd2.4\php\php.ini
      You can also run `php --ini` inside terminal to see which files are used by PH
    P in CLI mode.

解决：

        找到php.ini打开extension=php_mbstring.dll

报错：

    Deprecation Notice: The Composer\Package\LinkConstraint\MultiConstraint class is deprecated, use Composer\Semver\Constraint\MultiConstraint instead . in phar://C:/ProgramData/ComposerSetup/bin/composer.phar/src/Composer/Package/LinkConstraint/MultiConstraint.php:17

解决：

    composer global require "fxp/composer-asset-plugin:~1.0.0"
    修改成
    composer global require "fxp/composer-asset-plugin:~1.2.1"

报错:

    The openssl extension is required for SSL/TLS protection but is not available. If you can not enable the openssl extension, you can disable this error, at your own risk, by setting the 'disable-tls' option to true.

解决： 

    You should go to XAMPP php.ini and remove the semicolum from: extension=php_openssl.dll

参考：

    https://teamtreehouse.com/community/openssl-extension-is-required