I was seeing through stackoverflow the solutions for this kind of problems but any of them are executing the code of javascript in the same session...please i want some help with this...i have this in my session

<script>

function show_alert()
{    
    var ProdAntes = document.getElementById("productoseleccionado").value;
    var CantAntes = document.getElementById("cantidadantes").value;
    var PrecAntes = document.getElementById("precioantes").value;
    var FecAntes = document.getElementById("fechaantes").value;

    var ProdAhora = document.getElementById("SoyaProductorProduccionProducto").value;
    var CantAhora = document.getElementById("SoyaProductorProduccionCantidadtm").value;
    var PrecAhora = document.getElementById("SoyaProductorProduccionPreciodolar").value;
    var FecAhora = document.getElementById("select_date").value;
}
</script>

and in my html stuff i have this

<?php echo $this->Form->create('SoyaProductorProduccion');?>
    <fieldset>

        <?php 


        echo $this->Form->hidden('id', array('value' => $this->data['SoyaProductorProduccion']['id']));
        echo $this->Form->input('operacion', array('type' => 'hidden', 'value'=>'Produccion'));
        //--------------------------------------------------------------
        $productoseleccionado = $this->data['SoyaProductorProduccion']['producto'];
        echo $this->Form->input('productoseleccionado', array('type' => 'hidden','style'=>'width:500px; height:30px;','id' => 'productoseleccionado' , 'value' => $productoseleccionado));

        echo $this->Form->input('producto', array(
            'options' => array( 
                $productoseleccionado => $productoseleccionado,
                'Torta solvente de soya' => 'Torta solvente de soya',
                'Torta solvente de girasol' => 'Torta solvente de girasol',
                'Harina integral de soya' => 'Harina integral de soya',
                'Harina de girasol' => 'Harina de girasol',
                'Cascarilla de soya' => 'Cascarilla de soya',
                'Cascarilla de girasol' => 'Cascarilla de girasol',
                'Aceite de soya refinado' => 'Aceite de soya refinado',
                'Aceite de soya crudo' => 'Aceite de soya crudo',
                'Aceite de girasol refinado' => 'Aceite de girasol refinado',
                'Aceite de girasol crudo' => 'Aceite de girasol crudo',
                ),'label'=>'Tipo de Producto'
        ));



        foreach ($soyacambiodolares as $soyacambiodolar):
        $dolar=$soyacambiodolar['SoyaCambioDolar']['cambio'];
    endforeach;

    echo $this->Form->input('cambio', array('type' => 'hidden','value' => $dolar));
        //-----------------------------------------------------------------------------
        $cantidadantes = $this->data['SoyaProductorProduccion']['cantidadtm'];
        echo $this->Form->input('cantidadantes', array('type' => 'hidden','style'=>'width:500px; height:30px;', 'value' => $cantidadantes,'id' => 'cantidadantes'));

        echo $this->Form->input('cantidadtm', array('label' => 'Cantidad en tonelada(s) métrica(s) del producto  (TM)','style'=>'width:500px; height:30px;'));
        //-----------------------------------------------------------------------------
        $precioantes = $this->data['SoyaProductorProduccion']['preciodolar'];
        echo $this->Form->input('precioantes', array('type' => 'hidden','style'=>'width:500px; height:30px;', 'value' => $precioantes,'id' => 'precioantes'));

        echo $this->Form->input('preciodolar', array('label' => 'Precio en Dolares Americanos por tonelada métrica (TM / $us)','style'=>'width:500px; height:30px;'));
        //-----------------------------------------------------------------------------
        ?>
            <table style="width: 600px">
            <tr>
                <td >
                          <?php 
                          //----------------------------------------------------------------
                          $fechaantes = $this->data['SoyaProductorProduccion']['fecharegistro'];
                          echo $this->Form->input('fechaantes', array('type' => 'hidden','style'=>'width:500px; height:30px;', 'value' => $fechaantes, 'id' => 'fechaantes'));
                          //----------------------------------------------------------------

                          echo $this->Form->input("fecharegistro", array(
                                'label' => '<strong>Periodo al que corresponde la declaración</strong>', 'type' => 'text',
                              'style' => 'width: 110px', 
                              'class' => 'fl tal vat w300p', 'error' => false , 'id' => 'select_date')); 
                          ?>                 
                          <?php echo $this->Html->div('datepicker_img w100p fl pl460p pa',
                              $this->Html->image('datepicker_calendar_icon.gif'),array('id' => 'datepicker_img')); 
                          ?>
                          <?php echo $this->Html->div('datepicker fl pl460p pa', ' ' ,array('id' => 'datepicker')); 
                          ?> 
                </td>
            </tr>
        </table> 
            <?php 

        echo $this->Form->submit('Modificar Existencia', array('class' => 'form-submit',  'title' => 'Presione aqui para agregar datos', 'onclick' => 'return show_alert();')); 
?>
    </fieldset>
<?php echo $this->Form->end(); ?>

my function is ok but i want these:

when i click the submit button i want to compare wich field had been changed, and i want to create a chain of detailed changes like "change in the field 1, change in the fiel 2.--" and so on...and this has to be saved in my database so i have to pass to a variable in my php before saving...thanks!