Memory isn't something that we have to worry about very much in PHP, as memory management is handled for us by the Zend engine. However, when it does become an issue it becomes a very big one - most PHP script are limited as to how much memory they can consume. While this makes a lot of sense for web processes, and is in general not a problem, when you have a lot of data to deal with it can make life difficult.
If you asked most people how a search engine worked, their answer would likely be a far cry from the acres of servers and vast collections that Google queries millions of times a day. That said, the intuitive view of a search engine is in many ways just a series of incremental steps away from Mountain View.