How to change add-to-cart form? - woocommerce

Question

Is there a way to change the WooCommerce add-to-cart form through functions.php?
The goal is to add a checkbox for an additional product. When the checkbox is checked this product will also be added to the cart after a click on the add to cart button.
I am looking for a solution which doesn't rely on javascript.

Answer 1

A better title would be "WooCommerce up-sells as checkboxes".
A lot of research and several strategies to tackle this problem lead me to a solution which I thought was not even possible in the beginning.
The solution is now exactly what I wanted. A non-JavaScript, no-template-override, but a simple and pure addition to functions.php. It works for simple and variable products (and probably with grouped and external products too).
It misses some nice features still. It won't work yet if an up-sell is a variable product. Quantity selection and limiting up-sells per item or order would be nice additions too. Based on the code below adding those features should not be a big deal anymore.
// create the checkbox form fields and add them before the cart button
add_action( 'woocommerce_before_add_to_cart_button', 'action_woocommerce_before_add_to_cart_form', 10, 0 );
function action_woocommerce_before_add_to_cart_form(){
global $woocommerce, $product;
// get the product up-sells
$upsells = $product->get_upsells();
// store the number of up-sells and pass it on to the add-to-cart hook
?>
<input type="hidden" name="upsells_size" value="<?php echo(sizeof($upsells)); ?>">
<div id="wb-upsell-div">
<?php
// iterate through all upsells and add an input field for each
$i = 1;
foreach( $upsells as $value ){
$product_id = $value;
?>
<input id="wb-upsell-checkboxes" type="checkbox" name="upsell_<?php echo($i) ?>" value="<?php echo($product_id); ?>"><?php echo( '' . get_the_title( $product_id ) . "". " ($" . get_post_meta( $product_id, '_regular_price', true) . ")"); ?><br>
<?php
$i++;
}
?>
</div>
<?php
}
// function to add all up-sells, where the checkbox have been checked, to the cart
add_action('woocommerce_add_to_cart', 'custom_add_to_cart', 10, 3);
function custom_add_to_cart() {
global $woocommerce;
// get the number of up-sells to iterate through
$upsell_size = $_POST['upsells_size'];
// iterate through up-sell fields
for ($i=1; $i<=$upsell_size; $i++){
// get the product id of the up-sell
$product_id = $_POST['upsell_' . $i];
$found = false;
//check if product already in cart
if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
$_product = $values['data'];
if ( $_product->id == $product_id )
$found = true;
}
// if product not found, add it
if ( ! $found )
WC()->cart->add_to_cart( $product_id );
} else {
// if no products in cart, add it
WC()->cart->add_to_cart( $product_id );
}
}
}
And here is the CSS for formatting the <div>and the checkboxes. It goes into the style.css file:
#wb-upsell-div {
margin-top: 10px;
margin-bottom: 20px;
}
#wb-upsell-checkboxes{
}

Answer 2

So there's an actual answer to this question, you can add whatever you want inside the add to cart <form> using hooks. For example:
add_action( 'woocommerce_before_add_to_cart_button', 'so_34115452_add_input' );
function so_34115452_add_input(){
echo '<input type="checkbox" name="something"/>' . __( 'Some Checkbox', 'text-domain' );
}