Where to find structure of objects that I can access in PHP snippets added to different parts of UI? In views I can add PHP snippets for custom validation or when I want to provide default value for contextual filter. It is telling me that I can use $view or $argument - but where is documentation of these objects? Where can I read what are my possibilities in these areas (or how check this, by using debugger for example)?
You can install the devel module and use the dpm function to check it out.
You can check the whole list of devel functions at http://ratatosk.net/drupal/tutorials/debugging-drupal.html