For the production server, you probably want the best performance possible. Installing a PHP accelerator will give you the best improvement for your money.
APC is one of the most popular ones.
On a dedicated server
The set up is as simple as :
pecl install apc
On a shared hosting environment
Sometimes the installation is just not possible. Read the documentation.
If you use the Doctrine ORM
You can enable the cache system by adding the following method to
* Configure the Doctrine engine
public function configureDoctrine(Doctrine_Manager $manager)
$manager->setAttribute(Doctrine::ATTR_QUERY_CACHE, new Doctrine_Cache_Apc());