php: get option select value by mysql

I'm sorry for my english. I hope you to understand me. I'm trying to have an altas.php file for various html forms . so, I have this code in a html form with many other mysql columns:

<div><label for="categoria">Categoría principal</label><select type="text" name="categoria">
        <?php 
            $query = "select id, categoria from categorias";
            $result = mysqli_query($mysql,$query);
            if(!$result) echo 'Muy mal....';
            $num_filas = mysqli_num_rows($result);
            for ($i = 0; $i < $num_filas; $i++){
                $row = mysqli_fetch_array($result);
        ?>
        <option value "<?php echo $row['id']?>"><?php echo $row['categoria']?></option><?php
            }   
        ?>
        </select></div>

For the other hand, if I make this:

foreach($_POST as $campo => $valor){
        echo $campo ." = ". $valor ."\n";
    }

I have no results for categoria. Why not? I should have got the value, wich it's the categorias's id.

And something else: How can I use the "foreach" whith an insert sentence? For example:

 foreach($_POST as $campo => $valor){
$query ="INSERT INTO $tabla ($campo) VALUES ('$valor')";
}

$tabla is passed with $_GET. This code make a new row for each $campo, of course. Any idea? The most of the times, I find the answer to my questions, but this time, I'd try some suggestions, and nothing works.

Many thanks,