Creating a shortcode where you can show different filters

filter_products_for_woocommerce.php

<?php
/**
** Shortcode [box_filter]
** Author: Met El Idrissi - https://www.metelidrissi.com
** Creates a box where you can filter by WooCommerce Categories.
** To use this shortcode follow this steps:
** 	1 - Copy the entire copy and paste it inside your functions.pgp
**	2 - Open any page in your WordPress and paste the shortcode [box_filter]
**	3 - Remember to change the ID's for the respective ones you create on your WooCommerce Categories
**/

function box_shortcode_filter() {
    $dropdown = "";
    $args = array(
        'selected'           => 0,
        'hierarchical'       => 1,
        'taxonomy'           => 'product_cat',
        'hide_empty'         => false,
        'exclude_tree'            => 62,
        'exclude'            => 15, 62
    );

    $product_categories = get_terms( $args );
    $dropdown  .= "<select name='category'>";
    $dropdown  .= "<option value='categories'>Qualsevol categoría</option>";
    foreach ($product_categories as $category) {
        $dropdown  .= "<option  value = '" . esc_attr($category->slug) . "'>" . esc_html($category->name) . "</option>";
    }
    $dropdown  .= "</select>";

    /*
    ** Display attributes as a checkboxes. 
    ** These attributes are categories inside a generic one.
    ** In this case we assume the special category with ID 62
    */
	
    $checkbox = "";
    $checkbox .= "<div class='row'>";
    $checkbox .= "<div class='vc_column_container col-md-3'>" . woocommerce_subcats_from_parentcat_by_ID(63, 'Barrera') . "</div>";
    $checkbox .= "<div class='vc_column_container col-md-3'>" . woocommerce_subcats_from_parentcat_by_ID(68, 'Sellado') . "</div>";
    $checkbox .= "<div class='vc_column_container col-md-3'>" . woocommerce_subcats_from_parentcat_by_ID(72, 'Tratamiento') . "</div>";
    $checkbox .= "<div class='vc_column_container col-md-3'>" . woocommerce_subcats_from_parentcat_by_ID(79, 'Propiedades mecánicas') . "</div>";
    $checkbox .= "</div>";
	
    $checkbox .= "<div class='row'>";
    $checkbox .= "<div class='vc_column_container col-md-3'>" . woocommerce_subcats_from_parentcat_by_ID(85, 'Color') . "</div>";
    $checkbox .= "<div class='vc_column_container col-md-3'>" . woocommerce_subcats_from_parentcat_by_ID(93, 'Otras características') . "</div>";
    $checkbox .= "<div class='vc_column_container col-md-3'>&nbsp;</div>";
    $checkbox .= "<div class='vc_column_container col-md-3'>&nbsp;</div>";
    $checkbox .= "</div>";

    //Display the submit button for the form
    $submit = "";
    $submit .= "<input type='submit'>";

    $content = "";
    $content .= "<form action='filter_redirect.php' method='get'>";
    $content .= $dropdown . "<br><br><br>";
    $content .= $checkbox . "<br>";
    $content .= $submit;
    $content .= "</form>";

    return $content;
}

add_shortcode('box_filter', 'box_shortcode_filter');

/*
** Display subcategories by parent category ID.
*/

function woocommerce_subcats_from_parentcat_by_ID($parent_cat_ID, $title) {
    $args = array(
        'hierarchical' => 1,
        'show_option_none' => '',
        'hide_empty' => 0,
        'parent' => $parent_cat_ID,
        'taxonomy' => 'product_cat'
    );
    $subcats = get_categories($args);
    $checkbox = "";
    $checkbox .= "<b>". $title . "</b></br>";
    foreach ($subcats as $category) {
        $checkbox .= "<input type='checkbox' name='attributes[]' value='".$category->slug."' />" . esc_html($category->name) . "<br>";

    }
    return $checkbox;
}

add_action('get_categories_by_id', 'woocommerce_subcats_from_parentcat_by_ID');
?>