Yii: validation rules that always apply except one scenario

As of Yii 1.1.11 you can use except keyword:

array('username', 'exist', 'except' => 'update'),

Take a look at this page. There is a little example there.

Doc link

Work the same way in Yii 2.0.

['username', 'required', 'except' => 'update']

Every key in the array before the name of the validator is a property for the Validator Class itself. You can see the available properties in https://www.yiiframework.com/doc/api/2.0/yii-validators-validator

I know is an old question but every time I forgot that yii2 have an except property in the validator class.

https://www.yiiframework.com/doc/guide/2.0/en/input-validation for more advanced technics