Pass a query in a datatable to a modal

I want to visualize a detail of a datatable in a modal window from a datatable of a starting page but I can not visualize the output error. The code:

$query ="SELECT * FROM caja ORDER BY id DESC";  
$result = mysqli_query($connect, $query);
<table id="caja" class="table table-striped table-bordered">
                        <thead>
                          <tr>
                              <th>Id</th>
                              <th>Fecha</th>
                              <th>Compras</th>
                              <th>Ventas</th>
                              <th>Gastos</th>
                              <th>Total</th>
                              <th></th>
                          </tr>
                        </thead>
                        <tbody>
                         <?php  
                         $x=1;
                         //$c=substr($a,-8,2).substr($a,-5,2).substr($a,-2);
                          while($row = mysqli_fetch_array($result))  
                          {  
                               echo '  
                               <tr>  
                                    <td>'.$x++.'</td>
                                    <td>20'.substr($row["codigo"],-8,2).' - '.substr($row["codigo"],-4,2).' - '.substr($row["codigo"],-2).'</td>    
                                    <td>'.$row["compra"].'</td>  
                                    <td>'.$row["venta"].'</td>  
                                    <td>'.$row["gasto"].'</td>  
                                    <td>'.$row["total"].'</td>  
                                    <td> <button type="button" id="add_button" data-toggle="modal" data-target="#productModal" class="btn btn-info btn-lg" onclick="caja('.$row['codigo'].') ">Ver Detalle</button> <a data-toggle="modal" data-target="#productModal" href="#"?id='.$row['codigo'].' >Modificar</a></td> 
                               </tr>  
                               ';  
                          }  
                          ?>  
                        </tbody>
                    </table>

$querya ="SELECT * FROM cajadiaria where idcaja=".$row['codigo']." "; 
$resulta = mysqli_query($connect, $querya);

<div id="productModal" class="modal fade">
                                 <div class="modal-dialog">

                                   <div class="modal-content">
                                    <div class="modal-header">
                                     <button type="button" class="close" data-dismiss="modal">&times;</button>
                                     <h4 class="modal-title">FLUJO DE CAJA DIARIA</h4>
                                    </div>
                                    <div class="modal-body">
                                     <table id="cajadetalle" class="table table-striped table-bordered">
                                        <thead>
                                          <tr>
                                              <th>Id</th>

                                              <th>Compras</th>
                                              <th>Ventas</th>
                                              <th>Gastos</th>


                                          </tr>
                                        </thead>
                                        <tbody>
                                         <?php

                                         $a=1; 
                                          while($row = mysqli_fetch_array($resulta))  
                                          {  
                                             ?>

                                               <tr> 

                                                    <td><?php echo $a; $a++; ?></td>  

                                                    <td> <?php echo $row['compras']; ?></td>  
                                                    <td><?php echo $row['ventas']; ?></td>  
                                                    <td><?php echo $row['gasto']; ?></td>  


                                               </tr>  
                                            <?php     
                                          }  
                                            ?>
                                        </tbody>
                                        <tfoot>
                                            <tr>

                                                <th>Total:</th>
                                                 <th></th>
                                                  <th></th>
                                                   <th></th>
                                            </tr>
                                        </tfoot>
                                    </table>

                                    </div>

 <script>  
 $(document).ready(function(){  
      $('#caja').DataTable();  
 });  
 </script>
 <script>  
 $(document).ready(function(){  
      $('#cajadetalle').DataTable();  
 });  
 </script>