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I am really fond of PHPStorm. I moved from Notepad++ for most of my development a couple of years ago and have never looked back.
I don’t know just what specific support for YII2 it provides, but JetBrains is a supporter of YII.ORG.
It will really take a while to really start getting a lot of benefit out of a IDE, but I found just using it as an edited started showing me a lot of benefit.
I also hear that a lot of people are using netbeans for PHP as well. Someone else will need to sing its praises however, as I have never used it.
PhpStorm works great indeed, out of the box it recognizes most Yii2 objects and namespaces across multiple files and classes. There are also quite some extensions and plugins available for all JetBrains IDEs, and if you ever need to work on a Java/Python/etc. project, it’s easy to switch to another flavor of the JetBrains IDE.