如何使用现有的Symfony 2项目安装symfony cmf？

Question

我想将Symfony CMF安装到现有的Symfony2项目中。

我正在使用this documentation。

运行php composer.phar update后，出现以下错误：

[RuntimeException]

  Could not read http://packagist.org/packages-dev-n.json, either you or the remote host is probably offline.

  The "http://packagist.org/packages-dev-n.json" file could not be downloaded

update [--prefer-source] [--dry-run] [--dev] [--no-scripts] [packages1] ... [packagesN]

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我做错了什么？