PHP unable to retain answers during quiz review

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PHP

session

I have created a simple quiz in PHP.The user is able to log in.It displays one question per page.Then it goes for review.It allows the user to review one question per page.Each page has a next question button and previous question button(except first page).

Now I wish to retain already selected answers when I tap on previous question.

//code for Next question

if($submit=='Next Question' && isset($ans))
{
mysql_data_seek($rs,$_SESSION[qn]);
$row= mysql_fetch_row($rs);
mysql_query("insert into mst_useranswer(sess_id, test_id, que_des, ans1,ans2,ans3,ans4,true_ans,your_ans) values ('".session_id()."', $tid,'$row[2]','$row[3]','$row[4]','$row[5]', '$row[6]','$row[7]','$ans')") or die(mysql_error());
if($ans==$row[7])
{
$_SESSION[trueans]=$_SESSION[trueans]+1;
}

$_SESSION[yans][$p]=$ans;//$p is $_SESSION[qn]+1

$_SESSION[qn]=$_SESSION[qn]+1;
}

// code for previous question


if($_POST['prev']=='Previous Question')
{

$_SESSION[qn]=$_SESSION[qn]-1;
}

//code when i Load the quiz
if(isset($_SESSION[yans][$p]))
{
$a = $_SESSION[yans][$p]; // $p = $_SESSION[qn]+1
//echo $p;
}
echo "Que ". $n .": $row[2]</style>";
if($a==1)
{
echo "<input type=radio name=ans value=1 checked='checked'>$row[3] ";
}
else
{
echo "<input type=radio name=ans value=1>$row[3]";
}