stderr: PHP Warning: Creating default object from empty value

Question

(stderr: PHP Warning:  Creating default object from empty value in /ww_work/base/exam_class.php on line 1785,)

I'm no php programmer but apache logs are being filled with the creating default object from empty value array in the below function.

// this function put exam answer by student ito exm object.
// it also give the score to question if answer was correct.
public function put_answer($question_id,$answer_id){

    $this->given_answer[$question_id]=$answer_id;

    $question_obj = $this->questions[$question_id];
    //$answer_obj = new stdClass();
    $answer_obj = $question_obj->answer[$answer_id];
    $answer_obj->given_answer="Y";

    if(($answer_obj->given_answer==="Y")&&($answer_obj->correct_answer==="Y")){
        $question_obj->given_mark=$given_mark + $question_obj->score;
        $question_obj->given_answer=$answer_id;
        $question_obj->question_result="right";
    }else{
        $question_obj->given_mark=$given_mark + 0;
        //$answer_obj->given_answer="N";
    }

}

The error is being generated on the $answer_obj->given_answer="Y"; line.

Doing some googling i think this may be loosely connected (Creating default object from empty value in PHP?)

But if anyone has any ideas on how I can remove this warning, apart from turning off php warnings, it would be greatly appreciated.