behaviors.php

<?php
class UserBehavior exends \yii\base\Behavior
{
  public bool $myDynamicallyAddedProp = false;
}

$user = new \yii\web\User();
$user->attachBehavior('myUser', UserBehavior::class);

// I want to have $user be aware of myDynamicallyAddedProp, without having to "@var User|UserBehavior".
// We can't simply use @mixin on the \yii\web\User class, as that is a library class we don't control.
$user->myDynamicallyAddedProp;