title: Error in selecting multiple rows by checkbox error Undefined index: checkbox
body: <p>I want to select multiple rows from <code>checkbox</code> and want to update them all at once by button click..I tried in several ways..but not work..can you give a help please? it shows an error as <code>Undefined index: checkbox</code></p>
  
  <pre><code>&lt;td&gt;
      &lt;input type="hidden" name="id" id="id" value="&lt;?php echo $row["file_serial_id"]; ?&gt;" /&gt;
      &lt;input type="checkbox" name="checkbox[]" id="checkbox[]"  value="&lt;?php echo $row['file_serial_id']?&gt;" /&gt;
  &lt;/td&gt;
  
  if(isset($_POST['send_btn']) and $_POST['send_btn']=="Send"){
      $checkbox = $_POST['checkbox']; //from name="checkbox[]"
      $countCheck = count($_POST['checkbox']);
  
      for($i=0;$i&lt;$countCheck;$i++){
          $del_id = $checkbox[$i];
          $sql2 = "UPDATE retained_file_mst SET mark_to_dispose=1 WHERE file_serial_id='$del_id'";
          $result2 = mysql_query($sql2,$conn);
      }
  
      // if successful redirect to delete_multiple.php
      if($result2){
          echo "success";
      }else{
          echo "error";
      }
  }
  </code></pre>