$resISDEFE = mysqli_query($conexion,"SELECT * FROM personal_isdefe WHERE proyecto=".$idProyecto);
            if($resISDEFE->num_rows>0){
                    $index=0;
                    while($isdefe = mysqli_fetch_assoc($resISDEFE)){
                        $resCategoria = mysqli_query($conexion,"SELECT * FROM categoria_isdefe WHERE id=".$isdefe['categoria']);
                        $categoria = $resCategoria->fetch_row();
                        $idISDEFE = $isdefe['id']; <-- I save the row's ID here -->
                        echo "<tr id='isdefe'>";
                        echo "<td><p>".$idISDEFE."</p></td>";
                        echo "<td><p>".$categoria[1]."</p></td>";
                        echo "<td><p>".$isdefe['edg']."</p></td>";
                        echo "<td><p><a data-toggle='modal' data-target='#modalConcepto'>".$isdefe['porcentaje']."%</a></p></td>";
                        echo "<td><p>".$isdefe['horas_contratadas']."</p></td>";
                        $importe_prestacion = $isdefe['horas_contratadas'] * $categoria[2];
                        echo "<td><p>".number_format($importe_prestacion,2,",",".")." €</p></td>";

                        echo "<td><p>".number_format(($isdefe['importe_variable']+$importe_prestacion),2,",",".")." €</p></td>";
                        echo "</tr>";


                    }

<div class="modal fade" id="modalConcepto" role="dialog">
<div class="modal-dialog modal-lg">
  <div class="modal-content">
    <div class="modal-header">
      <h4 class="modal-title">Editar concepto</h4>
    </div>
    <div class="modal-body">
<div class="form-group">
  <textarea class="form-control" rows="10" id="newConcepto"></textarea><br>
</div>
    </div>
    <div class="modal-footer">
      <button type="button" class="btn btn-default" data-dismiss="modal" onclick='editConcepto(<?php $idISDEFE ?>)'>Guardar</button>
      <button type="button" class="btn btn-default" data-dismiss="modal">Cerrar</button>
    </div>
  </div>
</div>

function newConcepto(id, newConcepto){
            if(newConcepto!=""){
                $.get("ajax/editarConceptoISDEFE.php?id="+id+"&con="+newConcepto, function(data){
                        if(data=="1"){
                            var time = setTimeout(function(){
                                location.reload();
                            },500);
                        }
                        else alert("Error al actualizar el concepto");
                });
            }
    }

    function editConcepto(id){
        var newConcepto = $('#newConcepto').val();
        newConcepto(id,newConcepto);
    }
            }