How to fix the bug that is occuring in the script?

<?php
        if($_COOKIE['edited']) {
          session_start();
          }
        header("Cache-control: private"); //IE 6 Fix
        include "includes/lonconfig.php";
        $title = "Skill Trader Xur";
        include "includes/private_header.inc.php";
        if(!isset($_COOKIE['edited'])) {
          error("not_logged_in");
          return;
          }
/*       if($playerinfo[status]!=4) {
         echo "sorry we are closed for the moment<br> " ;
         return;
         }
*/
       $tendency=sin(date('j')*12)*.1;     //factor to affect prices to generate a ciclycal up/down tendency. I love this, jeje
       $stockdata=loaddata();
       $spread=.50;
       //echo "<br>$tendency";

switch ($action)
{
   case "buy";
      if ($amount!=1) { echo "What are you trying to do ? I didn't offer that deal!<br><br>"; echo "<center><a href=$PHP_SELF>Back</a><br></center>"; break; }
      if ($price!=ceil($stockdata[baseprice][$gem]*(1+$spread)))
      {
         echo "Gem Market prices have changed since last quote. New price is ".ceil($stockdata[baseprice][$gem]*(1+$spread))."<br>";
         echo "Do you want to continue the trade? (prices might change again till you decide)<br><Br>";
         echo "<a href=$PHP_SELF?action=buy&amount=$amount&gem=$gem&price=".ceil($stockdata[baseprice][$gem]*(1+$spread)).">Yes</a> / <a href=$PHP_SELF>No</a><br><br>";
         break;
      }

      if ($amount>$stockdata[amount][$gem]) { echo "Sorry, I don't have that much...<br>Come back later, maybe I can get some for you<br><br>"; echo "<center><a href=$PHP_SELF>Back</a><br></center>"; break; }

      if ($playerinfo[battle_points]>=$price*$amount)
      {
         echo "<br>You bought $amount ".$stockdata[gemname][$gem]."(s) for ".$price*$amount."<br><br>";
         echo "<center><a href=$PHP_SELF>Back</a><br></center>";
         mysql_query("update userdb set battle_points=battle_points-".$price*$amount.", ".$stockdata[gemname][$gem]."=".$stockdata[gemname][$gem]."+$amount where id=$playerinfo[id] limit 1");
         mysql_query("INSERT INTO `user_tradelogs` ( `fromid` , `type` , `itemdesc` , `amount` , `amount1` , `fecha` ) VALUES ( '$playerinfo[id]','13', '".$stockdata[gemname][$gem]."', '$amount', '".$price*$amount."', FROM_UNIXTIME(unix_timestamp())+0 )");

         $stockdata[baseprice][$gem]=1+ceil($stockdata[baseprice][$gem]*(1+$amount*.00005));
         $stockdata[amount][$gem]-=$amount;

         savedata($stockdata);
      } else echo "You don't have the required battle points to close that operation<br><br><center><a href=$PHP_SELF>Back</a><br></center>";
      break;

   default:

      echo "You enter the building and the amount of people screaming for buying and selling is amazing, You don't know where to go,";
      echo " Xur approaches you and tells you: \"Welcome to the Gem Market... Pretty busy, uh ?<br><br>What can I do for you?<br><br><br>";
      echo "<table width=100% border=1><tr><th>Item</th><th>Charging</th><th>Buy</th>";
      if ($playerinfo[status]==5) echo "<th>Stock</th>";
      echo "<th colspan=4>In Stock</th></tr>";

      for ($i=1;$i<=3;$i++)
      {
         $gemname=$stockdata[gemname][$i];
         $buyprice=ceil($stockdata[baseprice][$i]*(1-$spread));
         $amount=$stockdata[amount][$i];
         echo "<tr><td align=center>$gemname</td><td align=center>$buyprice</td>";
         if ($playerinfo[status]==5) echo "<td align=center>$amount</td>";
         echo "<td><a href=$PHP_SELF?action=buy&amount=1&gem=$i&price=$buyprice>Buy 1</a></td>";
         echo "<td>$amount</a></td>";
         echo "</tr>";
      }
      echo "</table>";

      echo "<center><font color=yellow>Battle Points: ($playerinfo[battle_points])</font></center>";
      echo "<br><br><center><a href=stxur.php>Exit Room</a><br></center>";

      break;
}


function loaddata()
{
   global $tendency;
   $filename='stocks.txt';
   $fp = @fopen($filename, "r");
   if($fp)
   {
      for ($i=1;$i<=3;$i++)
      {
         $stockdata[gemname][$i] =trim(fgets($fp));
          $stockdata[amount][$i] =(int)trim(fgets($fp));
          $stockdata[baseprice][$i] =(int)(trim(fgets($fp))*(1+$tendency));
          $stockdata[limit][$i] =(int)trim(fgets($fp));
       }
       fclose($fp);
       return $stockdata;
   }
}

function savedata($stockdata)
{
   global $tendency;
   $filename='stocks.txt';
   $fp = fopen($filename, "w+");
   if($fp)
   {
      for ($i=1;$i<=3;$i++)
      {
          fputs($fp,$stockdata[gemname][$i]."\n");
          fputs($fp,$stockdata[amount][$i]."\n");
          fputs($fp,round($stockdata[baseprice][$i]/(1+$tendency))."\n");
          fputs($fp,$stockdata[limit][$i]."\n");
       }
       fclose($fp);
   }
}
?>