<p><button type="button" title="<?php echo $this->__('Add to Cart') ?>" class="button btn-cart" onclick="setLocationAjax('<?php echo $this->getAddToCartUrl($_product) ?>','<?php echo $_product->getId()?>')"><span><span><?php echo $this->__('Add to Cart') ?></span></span></button></p>
 
<span id='ajax_loader<?php echo $_product->getId()?>' style='display:none'><img src='<?php echo $this->getSkinUrl('images/opc-ajax-loader.gif')?>'/></span>

http://www.excellencemagentoblog.com/ajax-based-product-add-to-cart-from-category-page

Magento Ajax Based Add to Cart from category page

ajax, magento

PHP