Magento: Checkout - Helper - Cart.php

<?php
//Path : app\code\local\ABC\Checkout\Helper\Cart.php
//http://magento.stackexchange.com/questions/45132/adding-a-product-to-the-cart-via-querystring-using-sku
class ABC_Checkout_Helper_Cart extends Mage_Checkout_Helper_Cart
{

    public function getProductIdByParams($param, $outputFlag = true){

        $sku = trim($param['sku']);
        $_product = Mage::getModel('catalog/product')->setStoreId(Mage::app()->getStore()->getId())
                                                     ->loadByAttribute('sku', $sku);

        if($_product){
            $productTypeId = $_product->getTypeId();
            $productId = $_product->getId();
            if($productTypeId == 'simple'){
                $result = $outputFlag ? true : $productId;
            }
            elseif($productTypeId == 'configurable'){
                $color = trim($param['color']);
                $size = trim($param['size']);
                $config = trim($param['option']);

                $configurableProduct = Mage::getModel('catalog/product_type_configurable')->setProduct($_product);
                $simpleCollections = $configurableProduct->getUsedProductCollection()
                                                         ->addAttributeToSelect('id');
                if(! empty($color)){
                    $attrColor = $_product->getResource()->getAttribute("sp_color");
                    $colorId = $attrColor->usesSource() ? $attrColor->getSource()->getOptionId($color) : '';
                    $simpleCollections->addAttributeToFilter('sp_color', $colorId);
                }
                if(! empty($size)){
                    $attrSize = $_product->getResource()->getAttribute("sp_size");
                    $sizeId = $attrSize->usesSource() ? $attrSize->getSource()->getOptionId($size) : '';
                    $simpleCollections->addAttributeToFilter('sp_size', $sizeId);
                }
                if(! empty($config)){
                    $attrConfig = $_product->getResource()->getAttribute("sp_config");
                    $configId = $attrConfig->usesSource() ? $attrConfig->getSource()->getOptionId($config) : '';
                    $simpleCollections->addAttributeToFilter('sp_config', $configId);
                }
                $simpleCollections->addFilterByRequiredOptions();

                if($simpleCollections->count() == 1){
                    $productId = $simpleCollections->getFirstItem()->getId();
                    $result = $outputFlag ? true : $productId;
                }
                else{
                    $result = false;
                }

            }
            return $result;
        }
        return false;
    }
}