Manage Admin columns WordPress

wp-admin-columns.php

/** Add columns **/
// Add to admin_init function
add_filter('manage_edit-productos_columns', 'add_new_productos_columns');
function add_new_productos_columns($columns) {
    $columns['cb'] = '<input type="checkbox" />';
    $columns['title'] = 'Producto';
    $columns['prod_ref'] = 'Referencia';
    $columns['prod_precio'] = 'Precio';
    $columns['image'] = 'Miniatura';

    return $columns;
}

// Add to admin_init function
add_action('manage_productos_posts_custom_column', 'manage_productos_columns', 10, 2);

function manage_productos_columns($column_name, $id) {
    global $wpdb;
    $custom = get_post_custom($id);

    if(isset($custom["prod_ref"][0]) ) $prod_ref = $custom["prod_ref"][0]; else $prod_ref ='';
    if(isset($custom["prod_precio"][0]) ) $prod_precio = $custom["prod_precio"][0]; else $prod_precio ="";

    switch ($column_name) {

    case 'prod_ref':
        echo $prod_ref;
        break;

    case 'prod_precio':
        echo $prod_precio;
        break;

    case 'image':
        echo get_the_post_thumbnail( $id, array(120,120) );
        break;
    default:
        break;
    } // end switch
}