ช่วยอธิบายโค้ดนี้หน่อยนะคับ ผมงงอ่ะ เกี่ยวกับ $a++

<?
$a=0;
while($a < $num_rows)
{
$result = mysql_fetch_array($db_query);
$id=$result[TC_ID];
$PASSWORD=$result[PASSWORD];


?>
<table width="99%" border="1" >
<tr> 
<td width="35%" bgcolor="#CCCCCC"> 
<div align="center" class="style89 style90"> 
  <div align="left">&nbsp;&nbsp;<?echo"$id";?></div>
</div></td>
<td width="38%" bgcolor="#CCCCCC"> 
<div align="center" class="style91">
  <div align="left">&nbsp;&nbsp;<?echo"$PASSWORD";?></div>
</div></td>
<td width="27%" bgcolor="#CCCCCC"> 
<div align="center" class="style91">
  <div align="center"><a href="TC1.1.php?id=<?echo"$TC_ID"; ?>">แก้ไข</a></div>
</div></td>
</tr>
</table>
<?
$a++;
}
?>