Add 'in stock' filter in product collection in magento

To get product that is in stock apply the below filter to your product collection -

$productDetails    = Mage::getModel('catalog/category')->load($cat_id)->getProductCollection()->addAttributeToSelect('*');
$productDetails -> addAttributeToFilter('visibility', array('in' => array(4) ));
$productDetails -> addAttributeToFilter('status', array('in' => array(1) ));
$productDetails->joinField('is_in_stock',
                'cataloginventory/stock_item',
                'is_in_stock',
                'product_id=entity_id',
                'is_in_stock=1',
                '{{table}}.stock_id=1',
                'left');      

Here we apply three filter - 'visibility' , 'status' , and finally our 'In stock' filter...