Description
This method first tries to find a cleaned-up submitted value, it will return a value set by setValue()/setDefaults()/setConstants() if submitted value does not exist for the given element.
The value that couldn't have possibly been submitted (e.g.: an option that is not present in <select> element's option list) is not considered valid and is not returned.
As the values returned by this method and by exportValues() are expected to be immediately processed and/or stored somewhere, values for file elements that obviously have special processing needs are not returned.
