input presensi

<html>
  <head>
  </head>
  <body>
    <table border = "1">
    <form method = "POST" action = "save_presensi.php">
    <?php
    date_default_timezone_set('Asia/Jakarta');
    $jam=date("H:i:s");
    $tanggal=date('Y-m-d');
    ?>
    <tr>
      <td colspan='7'>Tanggal :<input type="text" name="tanggal" id="tanggal" value="<?php echo $tanggal;?>" readonly></td>
    </tr>
    <tr>
      <th rowspan='2'>NIS</th>
      <th rowspan='2'>Nama</th>
      <th rowspan='2'>Kelas</th>
      <th colspan='4'>Keterangan</th>
    </tr>
    <tr>
      <th>Hadir</th>
      <th>Ijin</th>
      <th>Sakit</th>
      <th>Alfa</th>
    </tr>
    <?php
    include "connect.php";
    $id_kelas = $_GET['id_kelas'];
    $a = mysql_query("
    SELECT
      s.id_siswa ,s.nis, s.nama ,k.nama_kelas ,s.no_telp
    FROM
      tb_siswa s, tb_kelas k
    WHERE
      s.id_kelas = '$id_kelas'
      AND k.id_kelas = s.id_kelas
    ORDER
      BY s.id_kelas  ");
    $i=-1;
    while($b = mysql_fetch_array($a)){
      echo "
          <tr>
            <td><input type='text' name='data[".(++$i)."][nis]' value='{$b['nis']}' readonly></td>
            <td><input type='text'name='data[{$i}][nama]' value='{$b['nama']}' readonly></td>
            <td><input type='text'name='data[{$i}][nama_kelas]' value='{$b['nama_kelas']}' readonly></td>
            <td><input type='radio' name ='data[{$i}][ket]' value = 'hadir'></td>
            <td><input type='radio' name ='data[{$i}][ket]' value = 'ijin'></td>
            <td><input type='radio' name ='data[{$i}][ket]' value = 'sakit'></td>
            <td>
          <input type='radio' name ='data[{$i}][ket]' value = 'alfa'>
              <input type='hidden' name='data[{$i}][id_siswa]' value='{$b['id_siswa']}'>
              <input type='hidden' name='data[{$i}][id_kelas]' value='{$id_kelas}'>
            </td>
          </tr>";
    }
    ?>
    <tr>
      <td><input type="submit" name = "submit" value = "Selesai"></td>
    </tr>
    </form>
  </body>
</html>

//-- save_presensi.php
<?php
$datas=isset($_POST['data'])?$_POST['data']:array();
if(count($datas)>0){
  $value=array();
  foreach($datas as $data){
    $value[]="('{$data['nis']}','{$data['id_kelas']}','{$_POST['tanggal']}','{$data['ket']}')";
  }
  $sql="INSERT INTO tb_presensi (nis,id_kelas,tanggal,ket) VALUES ".implode(',',$value);
  include "connect.php";
  mysql_query($sql) or die('ada kesalahan '.mysql_error());
}
?>