Difference between values in while loop - php

Question

I am trying to compute the differences between array values in PHP in a while loop.
Initially I was using a foreach loop using (next) and (prev) but it didn't seem to work very well.
What I am trying now actually works but the problem is the value that is calculated should actually appear on the next line of the table.
Here is the table and HTML:
<table table id="meter_entries" class="table table-striped table-hover dt-responsive"><thead>
<tr>
<th>ID</th>
<th>Date</th>
<th>Company Code</th>
<th>Oil Height</th>
<th>Water Height</th>
<th>Total Volume</th>
<th>Difference</th>
<th>Actions</th>
</tr>
</thead>
<?php
$s = $stock_automatic; $i=0; $count = count($s);
while($i < $count){
?>
<tr>
<td><?php echo $s[$i]['id']; ?></td>
<td><?php echo $s[$i]['timess']; ?></td>
<td><?php echo $s[$i]['company_code']; ?></td>
<td><?php echo $s[$i]['oil_height']; ?></td>
<td><?php echo $s[$i]['water_height']; ?></td>
<td><?php echo $s[$i]['total_volume']; ?></td>
<td><?php $oldvol = $s[$i-1]['total_volume']; $currvol = $s[$i]['total_volume']; $diff = $oldvol - $currvol; echo $diff; ?></td>
<td>
<span class="fa fa-pencil"></span> Edit
<span class="fa fa-trash"></span> Delete
</td>
</tr>
<?php
$i++;
}
?>
</table>
Here is the output:

Answer 1

It seems that your problem is that you are using datatables and your array $s is actually ordered the opposite way than it is displayed in your example.
To get the correct values you should change your code to the following
<?php if ($i==$count-1){
echo '0';
}
else {
$oldvol = $s[$i+1]['total_volume']; $currvol = $s[$i]['total_volume'];
$diff = $currvol - $oldvol; echo $diff;
} ?>