when i submit data for the second time. the kode is a random number

 <?php
    include "koneksi.php";
    $data = "SELECT max(nomor) as nomor from tbl_permohonan";
    $sql = mysql_query($data);
    if($sql === FALSE) { 
    die(mysql_error()); // TODO: better error handling
  }
  $i= mysql_fetch_array($sql);
// ID OTOMATIS//***************************************************
  $kodeawal=substr($i['nomor'],2,7)+1;
  if($kodeawal<10){
    $kode='190281000'.$kodeawal;
  }elseif($kodeawal > 9 && $kodeawal <=99){
    $kode='19028100'.$kodeawal;
  }else{
    $kode='1902810'.$kodeawal;
  }
  ?>

I think I wrong on $kodeawal=substr($i['nomor'],2,7)+1;, but I dont know where is the bug. So help me fix it. Thanks !