if loop not working!

[peter162in]

Please help something wrong I am a new b in programming.

<?php

include "dbcon1.php";

$sql = "SELECT date, price  FROM `table1` ORDER BY table.date DESC";
$result = mysql_query($sql) 
  or die(mysql_error());
$my_header=<<<EOD

  <h2><center>PROFIT ANALYSIS<font color="red" size="6"></h2>
  <table width="70%" border="1" cellpadding="2"
         cellspacing="2" align="center">
    <tr>
         <th>DATE</th>
         <th>PRICE</th>
   <th>DIFF</th>

           
    </tr>
     
EOD;
$my_details = '';

  

$priordate = '';
$priorprice = null;

while ($row = mysql_fetch_array($result)) {
  $date = $row['date'];
  
  $price = $row['price'];

  if ($priordate == '') {
     $priordate = $date;
   echo $priordate; 
     $priorprice = $price;
echo $priorprice;
    }
echo $priorprice;// price captured remain as initial price
   $diff = $priorprice - $price; //PRIORPRICE($priorprice) REMAIN UNCHANGED AS DATE CAPTURED REMAIN THE INITIAL DATE
    $my_details .=<<<EOD
   <tr>
  <td>$date</td>
        <td><font color="red" size="4"> $price</td>
   <td><font color="red" size="4"> $diff</td>
           
  </tr>
EOD;
}
$my_footer ="</table>";     
     
$final =<<<RESULT
               $my_header
               $my_details
               $my_footer
RESULT;

  echo $final;
?>

MY RESULT

 

DATE        PRICE    DIFF

2009-08-01      1232  0

2009-08-04      1250  18

2009-08-05      1273  41

2009-08-19      1181  -51

PRICE DIFF DISPLAYED FROM THE START DATE

 

ACTUAL RESULT I EXPECT IS

DATE        PRICE    DIFF

2009-08-01      1232  0

2009-08-04      1250  18

2009-08-05      1273  23

2009-08-19      1181  -92

THE DATE IS NOT CAPTURING . PLEASE HELP ME. if loop ?