How to pass the product parameter when clicking on the add to cart button?

Question

PHP

How to pass the product parameter when clicking on the add to cart button? the button for adding a product is in the category output template category.tpl 0.5 kg=1pc 1kg=2pcs etc.

<div class="add">
                                    <select>
                                        <option>0.5 кг</option>
                                        <option>1 кг</option>
                                        <option>1.5 кг</option>
                                        <option>2 кг</option>
                                        <option>2.5 кг</option>
                                        <option>3 кг</option>
                                        <option>3.5 кг</option>
                                        <option>4 кг</option>
                                    </select>
                                     <button type="button" onclick="cart.add('<?php echo $product['product_id']; ?>');">
                                        <span class="price" >
                                            <?php echo $product['price']; ?>
                                        </span>
                                    </button>
                                </div>

Answer 1

The question is not entirely clear, and I'm not strong in opencart either. However...
The function accepts a second argument (quantity):

onclick="cart.add('<?php echo $product['product_id']; ?>', $('select').val());"
/*
тут важно получать значение нужного селекта
*/

If you are going to pass some other parameter, then you need to find this function and change it a little.

'add': function(product_id, quantity) {
  var dopparam = $('тут что-то получаем').val();
  $.ajax({
    url: 'index.php?route=checkout/cart/add',
    type: 'post',
    data: 'product_id=' + product_id + '&quantity=' + (typeof(quantity) != 'undefined' ? quantity : 1) + '&dopparam=' + dopparam,
    dataType: 'json',
    beforeSend: function() {
      $('#cart > button').button('loading');
    },
    success: function(json) {}
}

Only then do not forget to explain to the basket what to do with this additional parameter.