Problem with php page

I set up a very rudimentary pager, when entering my index.php page a query of all the records. To do this, I only pass the page number to show the url and reload the same page, which works.

Now I have the need to show and paginate the records by category. I put a select in my index.php and when I choose category I pass category + '1' to another page called categoria.php where I have the same functions as in my index (I copy how this value happened):

<select id="combo_categorias" name="cat" onchange="location.href='categoria.php?num=' + this.value +'1';">

I control by url the value of the category that is a string, eg: 'shoe shop' + a 1, since the first page to be displayed is always 1.

The issue is that already stopped in categoria.php , when I have paginada more than 1 page does not work, I can pass the value of the page .. but the value of the category is lost and I can not find the way to solve it. Copy the code:

   <?php  
        $variable=" ";
        $rest=" ";
        $restvar=" ";

        //Si lo recibido por GET
        if (isset($_GET['num'])) {
            $variable=$_GET['num'];
        }

        //Saco del GET el último dígito
            $rest=substr($variable, -1);

        //Saco todo menos el último dígito
            $restvar=substr($variable, 0, -1);

        //Convierto lo sustraído en la variable
            $variable=$restvar;
//            $_SESSION["cat_elegida"]=$variable;
    ?>

    <div class="pagina">
        <?php include ("cabecera.php"); ?>
            <div class="contenedor">
                    <div class="productos">
        <?php

        $consulta="select * from productos where categoria LIKE '$variable' ORDER BY id ASC";
        $result = mysqli_query($con, $consulta);
        $nro_reg = mysqli_num_rows($result);

        if ($nro_reg==0) {
            ?>    
                <div class="superado_stock">    
                <?php
                    echo "<h4> </h4>";
                    echo "<h4><strong>Producto temporalmente agotado</strong></h4>";
                    echo "<h4> </h4>";
                ?>
                </div>                               
                <?php
            }

        $reg_por_pagina=12;

        if (isset($_GET['num'])) {
            $nro_pagina=$rest;
            var_dump($nro_pagina);
            var_dump($variable);
        } else {
            $nro_pagina=1;
        }

        if (is_numeric($nro_pagina))
            $inicio=(($nro_pagina-1)*$reg_por_pagina);
        else
            $inicio=0;

        $consulta=mysqli_query($con,"select * from productos where categoria LIKE '$variable' order by id ASC limit $inicio, $reg_por_pagina");
        $canti_paginas=ceil($nro_reg/$reg_por_pagina);
        ?>
        <?php
            while ($filas=mysqli_fetch_array($consulta, MYSQLI_ASSOC)) {
            echo mysql_error();
                $id=$filas['id'];
                $imagen=$filas['imagen'];
                $nombre=$filas['nombre'];
                $descripcion=$filas['descripcion'];
                $precio=$filas['precio'];
                $stock=$filas['stock'];
                $fecha=$filas['fecha'];
                if($imagen=="No disponible"){
                    $imagen="IMAGENES/ImagenNoDisponible100.gif";
                }                
        ?>

            <div class="caja">
                <div class="caja_imagen"><img src="<?php echo $imagen?>" style=widht="100" height="100"/>
                        <p class="caja_nombre"><?php echo $nombre?></p>
                        <p class="caja_precio">$ <?php echo $precio?></p>
                </div>   
                <div class="caja_boton">
                    <form action="detalle.php" method="post" name="detalle">
                        <input name="id" type="hidden" value="<?php echo $id ?>" />
                        <input class="boton_detalle2" type="submit" value="Detalle">
                    </form>
                </div>
            </div>
       <?php
            }
        ?>
            </div>
        </div>       
    <!-------------------------------------------------------------------------------------------->
    <!-----------------------------PAGINADOR------------------------------------------------------>
    <!-------------------------------------------------------------------------------------------->      

            <div id="paginador" align="center">
                        <?php
                        if ($nro_pagina>1)
                            echo "<a href='categoria.php?num=".($nro_pagina-1) ."' >Anterior</a> ";

                        for ($i=1;$i<=$canti_paginas;$i++){

                            if ($i==$nro_pagina)
                                echo "<span>$i </span>" ;
                            else
                                echo "<a href='categoria.php?num=$i'>$i</a> ";
                        }

                        if ($nro_pagina<$canti_paginas)
                            echo "<a href='categoria.php?num=". ($nro_pagina+1) ."' >Siguiente</a> ";
                        ?>
            </div>
    <!-------------------------------------------------------------------------------------------->
    <!--------------------------FIN DEL PAGINADOR------------------------------------------------->
    <!-------------------------------------------------------------------------------------------->   
        </div>   
    </div>

Maybe I could pass 2 values x url, but I do not succeed.