i want to calculate subtraction of row2 & row1 - php

Question

I display mysql table data using php.
I search but find solution for column but not for row.
Below I try to show what I want...
<?php
$i=0;
while($row = mysqli_fetch_array($result)) {
?>
<table>
<tr>
<td id="<?php echo $row["id"]; ?>"><?php echo $row["total"]; ?></td>
<td id="difference"> Difference from previous row. </td>
</tr>
<?php
$i++;
}
?>
</table>

Answer 1

<?php
$i=0;
$oldval = 0;
while($row = mysqli_fetch_array($result)) {
?>
<table>
<tr>
<td id="<?php echo $row["id"]; ?>"><?php echo $row["total"]; ?></td>
<td id="difference"> <?php echo ($i==0) ? $oldval : $row['total']-$oldval; ?> </td>
</tr>
<?php
$oldval = $row["total"];
$i++;
}
?>
</table>
$oldval variable is used to store current row's total field data so when you go to the next row, you can get the difference for current row because you have previous row's data stored in the $oldval
And the ternary condition I put there is because if $i==0 means the first row so you don't have any data of the previous row so by default difference is 0 and you can notice that I stored the current record's total field data after printing the difference