Despite the fact that you may not have a choice in the matter, upgrading comes with a number of bonus new features that can help you write better code and gain access to new functionality that required a fair amount of hacking in previous version. Here's a quick list of 5 personal favourites.

The feature to make his list are SimpleXML, JSON/SOAP, PDO, the Standard PHP Library and SQLite. Each has their own bonus feature(s) included too for a little extra incentive to check them out.

Physical memory was limited and Apache and MySQL soon started to fight for every remaining byte and eventually the page was unavailable. After this experience I started to look for a alternative for the database storage and I found an interesting one: SQLite.

He mentions some of the features the database has including zero-configuration, a simple API, and the portability of having a single database in a single file. The next step was getting it to work with his Apache/PHP5 installation and to run some statistics on it. The results show SQLite pulling ahead of MySQL (INNODB) by a bit and MySQL (MYISAM) by a good bit.