How to display error message inside a pop up window instead of moving to a new page?

How to display error message inside a pop up window instead of moving to a
new page?

I'm new to php, and I'm new here and I don't know if it's right for me to
ask a question in such a long manner. I'm sorry if that's not how it is
done here.
I have the following code which is used to enter a question and it's four
choices.
4 choices are entered in text boxes with a radio button corresponding to
each.
The button clicked out of those 4 will be the value of $correct
$difflevel, $agegroup and $catId are again, the values from a set of radio
buttons.
The following is my code
if(isset($_POST['submit']))
{
extract($_POST);
$question=mysqli_real_escape_string($con,$question);
$option[0]=mysqli_real_escape_string($con,$option[0]);
$option[1]=mysqli_real_escape_string($con,$option[1]);
$option[2]=mysqli_real_escape_string($con,$option[2]);
$option[3]=mysqli_real_escape_string($con,$option[3]);
mysqli_query($con,"insert into questions(question,
op1,op2,op3,op4,correct,difflevel,ageGroup,catId) values
('$question','$option[0]','$option[1]','$option[2]','$option[3]',$correct,$difflevel,$ageGroup,$catId)")
or die("Some error");
$questionId = mysqli_insert_id($con);
if($_POST['catId']=="")//QUESTIONS MUST CONTAIN AT LEAST ONE CATEGORY
die("Please select category of the question entered.");
if($_POST['difflevel']=="")//DIFFICULTY LEVEL SHOULD BE SET
die("Please select difficulty of the question entered.");
if($_POST['ageGroup']=="")//AGEGROUP SHOULD BE SET
die("Please select age group of the question entered.");
if($_POST['correct']=="")//CORRECT ANSWER SHOULD BE SET
die("Please select correct answer for the question entered.");
}
Since all fields are mandatory, I need an error report if not all are
filled(or clicked). Right now, when the error occur, it's taken to a new
page, and hence all that I typed are lost and I have to type them again.
So instead of going to the next page, is it possible to show a pop-up
window with the error so that I can correct the mistake and continue from
where I stopped? Any help is so much appreciated. Thanks in advance.