Read the source. From woocommerce-ajax.php (in the plugin’s root folder)
Javascript/Ajax add to cart
/**
* AJAX add to cart
*
* @access public
* @return void
*/
function woocommerce_ajax_add_to_cart() {
global $woocommerce;
check_ajax_referer( 'add-to-cart', 'security' );
$product_id = (int) apply_filters('woocommerce_add_to_cart_product_id', $_POST['product_id']);
$passed_validation = apply_filters('woocommerce_add_to_cart_validation', true, $product_id, 1);
if ($passed_validation && $woocommerce->cart->add_to_cart($product_id, 1)) :
// Return html fragments
$data = apply_filters('add_to_cart_fragments', array());
do_action( 'woocommerce_ajax_added_to_cart', $product_id);
else :
// If there was an error adding to the cart, redirect to the product page to show any errors
$data = array(
'error' => true,
'product_url' => get_permalink( $product_id )
);
$woocommerce->set_messages();
endif;
echo json_encode( $data );
die();
}
add_action('wp_ajax_woocommerce_add_to_cart', 'woocommerce_ajax_add_to_cart');
add_action('wp_ajax_nopriv_woocommerce_add_to_cart', 'woocommerce_ajax_add_to_cart');
Non-AJAX: Requires Page Load
If you are not using AJAX then you are using the woocommerce_add_to_cart_action() from woocommerce-functions.php which runs on the init hook. It is a bit long, so I will just let you read it from the mentioned file. Basically it looks for an add-to-cart` query parameter (which should equal the product ID) and goes from there.