Menampilkan data sesuai waktu

Assalamualaikum Warahmatullahi Wabarakatuh...
 
pm line : farreldivaanelca
berikut kodingan nya 
 
 <?php 
  $i=1;
 
 
  do {
    $id = $row_inv['id_inv'];
    $sum = "SELECT SUM(nilai) AS Kuda FROM nilai_inv WHERE id_inv=$id";
    $invs = mysql_query($sum, $koneksi) or die(mysql_error());
    $co = mysql_fetch_assoc($invs);
 
    $ss = "SELECT tanggal, SUM(nilai) as nilai from nilai_inv WHERE id_inv=$id GROUP BY tanggal";
    $iss = mysql_query($ss, $koneksi) or die(mysql_error());
 
 
 
    while ($cd = mysql_fetch_assoc($iss)) {
    $tgl = new DateTime($cd['tanggal']);
    $date = new DateTime();
    $diff = $tgl->diff($date);
    $waktu = $diff->days;
    $o = 0;
    if($waktu<7){
      $tujuh[$o] = $cd['nilai'];
    }elseif ($waktu<15) {
      $mablas[$o] = $cd['nilai'];
    }elseif ($waktu<30) {
      $gapulu[$o] = $cd['nilai'];
    }else{
      $tuga[$o] = $cd['nilai'];
    }
  }
 
    ?>
    <tr>
      <td align="center"><?php echo $i;$i++ ?></td>
      <td align="center"><?php echo $row_inv['kepada']; ?></td>
      <td align="center"><?php echo $co['Kuda'];?></td>
      <td align="center"><?php echo $tujuh[$o];?></td>
      <td align="center"><?php echo $mablas[$o]; ?></td>
      <td align="center"><?php echo $gapulu[$o]; ?></td>
      <td align="center"><?php echo $tuga[$o]; ?></td>
    </tr>
    <?php } while ($row_inv = mysql_fetch_assoc($inv));
    ?>
 
saat ditampilkan terjadi error seperti di gambar