Magento difference between Mage::getModel() and Mage::getSingleton()

Mage::getModel() will always return a new Object for the given model:

/**
 * Retrieve model object
 *
 * @link    Mage_Core_Model_Config::getModelInstance
 * @param   string $modelClass
 * @param   array|object $arguments
 * @return  Mage_Core_Model_Abstract|false
 */
public static function getModel($modelClass = '', $arguments = array())
{
    return self::getConfig()->getModelInstance($modelClass, $arguments);
}
Mage::getSingleton() will check whether the Object of the given model already exists and return that if it does. If it doesn't exist, it will create a new object of the given model and put in registry that it already exists. Next call will not return a new object but the existing one:

/**
 * Retrieve model object singleton
 *
 * @param   string $modelClass
 * @param   array $arguments
 * @return  Mage_Core_Model_Abstract
 */
public static function getSingleton($modelClass='', array $arguments=array())
{
    $registryKey = '_singleton/'.$modelClass;
    if (!self::registry($registryKey)) {
        self::register($registryKey, self::getModel($modelClass, $arguments));
    }
    return self::registry($registryKey);
}
In your case you always want a completely new Product object/model since every product is unique.

Extending / Overriding Magento controller

We can override any controller using our custom extension, so you will need to create below extension files and folders:-

For ex. here we are overriding checkout cart controller.

• app/etc/modules/Namespace_YourModuleName.xml: It's a file used to enable our custom module.
• app/code/local/Namespace/YourModuleName/etc/config.xml: It's a module configuration file in which we'll set up controller class overriding using certain tags as per the Magento conventions.
• app/code/local/Namespace/YourModuleName/controllers/CartController.php: It's a custom controller class file which we'll use to override the core "Cart" controller class.

In your app/etc/modules/Namespace_YourModuleName.xml file, write below code:

<?xml version="1.0" encoding="UTF-8"?>

<config>

<modules>

<Namespace_YourModuleName>

<active>true</active>

<codePool>local</codePool>

</Namespace_YourModuleName>

</modules>

</config>

In your app/code/local/Namespace/YourModuleName/etc/config.xml file, write below code:

<?xml version="1.0" encoding="UTF-8"?>

<config>

<modules>

<Namespace_YourModuleName>

<version>0.1.0</version>

</Namespace_YourModuleName>

</modules>

<frontend>

<routers>

<checkout>

<args>

<modules>

<Namespace_YourModuleName before="Mage_Checkout">Namespace_YourModuleName</Namespace_YourModuleName>

</modules>

</args>

</checkout>

</routers>

</frontend>

</config>

In your app/code/local/Namespace/YourModuleName/controllers/CartController.php file, write below code:

<?php

require_once 'Mage/Checkout/controllers/CartController.php';

class Namespace_YourModuleName_CartController extends Mage_Checkout_CartController

{

public function addAction()

{

echo 'I successfully Override Cart Controller';

parent::addAction();

}

public function indexAction()

{

echo 'I successfully Override Cart Controller';

parent::addAction();

}

}

?> 

Magento : Get Skin Url, Base Url , Media Url , Store Url , Js Url , and Current Url

Get Skin Url :-

Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_SKIN);

Get Base Url :

Mage::getBaseUrl();

Secure Skin Url :

$this->getSkinUrl('images/imagename.gif', array('_secure'=>true));

Unsecure Skin Url :

$this->getSkinUrl('images/imagename.jpg');

Get Media Url :

Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_MEDIA);

Get Js Url :

Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_JS);

Get Store Url :

Mage::getBaseUrl(Mage_Core_Model_Store::URL_TYPE_WEB);

Get Current Url

Mage::helper('core/url')->getCurrentUrl();

Get Url in cms pages or static blocks

Get Skin Url :

{{skin url='images/imagename.jpg'}}

Get Base Url :

{{store url=""}}

Get Media Url :

{{media url='/imagename.jpg'}}

Get Store Url :

{{store url='mypage.html'}}