PHP NOTICE:Undefined offset 1 - php

Question

I am facing a problem that undefined offset :1. I can't understand that what type of error it is. Can anyone tell me that why such error occurs in php
<?php
$host="localhost";
$dbusername="root";
$dbpassword="";
$dbname="school";
$conn=mysql_connect($host,$dbusername,$dbpassword) or die(mysql_error());
mysql_select_db($dbname) or die(mysql_error());
if(isset($_POST['submit']))
{
$cls=$_POST['hhcls'];
{
{
{
$query=mysql_query("select * from stu_class where class='$cls'",$conn);
while($data=mysql_fetch_row($query))
{
$r1=$data[0];
$r=$_POST[$r1]; //THIS LNE ERROR
echo $r;
$query1="update stu_class set rollno=".$r." where ad_no=".$r;
$sql_query1=mysql_query($query1) or die(mysql_error());
if($sql_query1)
{
echo "<script type='text/javascript'>alert('Students Details Updated Successfully');</script>";
}
else
{
echo "<script type='text/javascript'>alert('Updation Failed');</script>";
}
}
}
}
}
}
?>
I am getting error while passing data to another query

Answer 1

Of course )
Look:
$query = mysql_query("select * from stu_class where class='$cls'", $conn);
// Now we got not query, but result of this query in $query
while ($data = mysql_fetch_row($query)) {
// Every iteration we will have in $data array (not associated)
// of fields (next row from database)
$r1 = $data[0];
// After this line we have value of first column of current line in $r1
// usually this is integer ID field and if first ID in database is 1,
// then $r1 = 1
$r = $_POST[$r1]; //THIS LNE ERROR
// here you trying to set $r = $_POST[1] because $r1 = 1
// but you have no any POST variable with key 1.
When you trying to get some array member that doesn't exist, you will get notice "Undefined offset %key%". In strict mode you will get exception.