Yii framework - what is the file ".yii" in some folders

There is ".yii" file in the following folders:

• /assets/
• /images/
• /protected/commands/shell/
• /protected/extensions/
• /protected/messages/
• /protected/runtime/
• /protected/views/system/
• /themes/classic/views/layouts/
• /themes/classic/vi开发者_高级运维ews/site/
• /themes/classic/views/system/

What is it?

I believe they are generated by yiic, not sure what purpose they have though as they always appear to be empty unless they are purely used as a means of storing a timestamp against the build date.

i assumed it was some kind of placeholder to stop mercurial/git from ignoring the directory