I believe that the above code is setting the first element of questionArray to the array $ob->questionArray.
I'm pretty sure you have to set each element specifically:
Code:
<input type=hidden name=questionArray[0] value=$ob->questionArray[0]>
<input type=hidden name=questionArray[1] value=$ob->questionArray[1]>
etc. You could pretty easily build a foreach loop that did this for you.