Coming from SublimeText, I missed opening an editor window and start typing or copying and pasting content that I would throw away later. With PHPStorm, you have to create a new file under the project. However, a handy plugin comes to the rescue.
Scratch Plugin
Install the Scratch plugin for PHPStorm. The plugin allows you to open an editor window which is not part of the PHPStorm project. You can either re-use the existing scratch files or create one.
Enjoy!