Differences between PHP 4 and PHP 5

While PHP 5 was purposely designed to be as compatible as possible with previous versions, there are some significant changes. Some of these changes include:

A new OOP model based on the Zend Engine 2.0
A new extension for improved MySQL support
Built-in native support for SQLite
A new error reporting constant, E_STRICT, for run-time code suggestions
A host of new functions to simplify code authoring (and reduce the need to write your own functions for many common procedures)

For more detailed information, please view the section on Migrating from PHP 4 to PHP 5 and the section on Backwards Incompatible Changes.