Hi, I am having problem in my update,how can i add like this number in php

lets assume that the value of my post is "00000010"

how can i add so that it will be like "00000011" and then the next update will be "00000012" and etc....

my code doesn't work it will give only "1"

Code:
   
   $num = $_POST['bookno'];

   $num = $num + 1;
    
    $sql = "update mytable set `bkno` = '$num' where `bkcode` = '25'";

    $result = mysql_query($sql);

    if(!$result){
      die("failed to update".mysql_error());
  }

Can you help me please.Thank you in advance.