Warning: Division by zero

<center>
<form action="" method="post">
RuneScape Name: <input name="username" value="<?=$_POST["username"]?>" type="text" maxlength="12" size="12">
<input type="submit" value="Look Up" name="sub">
<?php
  if($_POST["sub"]=="Look Up") {
?>
<hr>  
<b>Personal scores for <?=$_POST["username"]?></b>
<hr>
</center>
<?php
if($_POST["sub"]=="Look Up") {
  $username = strtolower($_POST["username"]);
  $level = file_get_contents("http://hiscore.runescape.com/index_lite.ws?player=$ username");
  $rank = file_get_contents("http://hiscore.runescape.com/index_lite.ws?player=$ username");
  $xp = file_get_contents("http://hiscore.runescape.com/index_lite.ws?player=$ username");
  function dajlvl($trojka) {
  $trojka = explode(",",$trojka);
  return $trojka[1];
  }
  function dajxp($trojka) {
  $trojka = explode(",",$trojka);
  return $trojka[2];
  }
  function dajrank($trojka) {
  $trojka = explode(",",$trojka);
  return $trojka[0];
  }
  $level = explode("\n",$level);
  $rank = explode("\n",$rank);
  $xp = explode("\n",$xp);
//-------------------------------------------------------------------- ----
  function experience($L) {
    $a=0;
    for($x=1; $x<$L; $x++) {
      $a += floor($x+300*pow(2, ($x/7)));
    }
    return floor($a/4);
  }
  $attack_1= dajlvl($level[1]) + 1;
  $attack_xp1=experience($attack_1);
  $attack_2= dajlvl($level[1]);
  $attack_xp2=experience($attack_2);
//__________attack__________\\
  $attack_lvl = dajlvl($level[1]);
  $attack_rank = number_format (dajrank($rank[1]),0,'.',' ');
  $attack_xp = number_format (dajxp($xp[1]),0,'.',' ');
  $attack_progress = round(100-((($attack_xp1 - dajxp($xp[1])) / ($attack_xp1 - $attack_xp2))*100));
  $attack_next = number_format ($attack_xp1 - dajxp($xp[1]),0,'.',' ');

  if(dajlvl($level[1]) == -1) $attack_lvl = "1";
  if(dajlvl($level[1]) == -1) $attack_rank = "Not Ranked";
  if(dajlvl($level[1]) == -1) $attack_xp = "- -";
  if(dajlvl($level[1]) == -1) $attack_progress = "0";
  if(dajlvl($level[1]) == -1) $attack_next = "- -";
  if(dajlvl($level[1]) == 99) $attack_next = number_format (13034431 - dajxp($xp[1]),0,'.',' ');
  if(dajlvl($level[1]) == 99) $attack_progress = 100;
}
?>
&nbsp;
<!-------------------- Skills -------------------->
<table border="1" cellspacing="0" cellpadding="2" align="center" style="border-collapse: collapse"
bordercolor="#222222">
  <tr>
    <td colspan="2" align="center">Skill</td>
    <td align="center">Level</td>
    <td align="center">Rank</td>
    <td align="center">XP</td>
    <td align="center">Progress To Next Level</td>   
    <td align="center">XP To Next Level</td>
  </tr>
  
  <tr title="Attack">
    <td align="center" title="icon"><img src="img/skill_ico/attack.gif"></td>
    <td align="left" title="skill">Attack</td>
    <td align="center" title="level"><?=$attack_lvl?></td>
    <td align="center" title="rank"><?=$attack_rank?></td>
    <td align="center" title="xp"><?=$attack_xp?></td>
    <td align="center" title="progress"><img src="img/bar/<?=$attack_progress?>.gif" border="0"></td>
    <td align="center" title="xp to next lvl"><?=$attack_next?></td>
  </tr>
</table>
</center>
<?php
  }
?>
</form>