Some of the key features of PHP 5 include:

The Zend Engine II with a new object model and dozens of new features.
XML support has been completely redone in PHP 5, all extensions are now focused around the excellent libxml2 library (http://www.xmlsoft.org/).
A new SimpleXML extension for easily accessing and manipulating XML as PHP objects. It can also interface with the DOM extension and vice-versa.
A brand new built-in SOAP extension for interoperability with Web Services.
A new MySQL extension named MySQLi for developers using MySQL 4.1 and later. This new extension includes an object-oriented interface in addition to a traditional interface; as well as support for many of MySQL's new features, such as prepared statements.
SQLite has been bundled with PHP. For more information on SQLite, please visit their website.
Streams have been greatly improved, including the ability to access low-level socket operations on streams.
And lots more...
PHP5加入了最新的ZENDII引擎；

对XML完美支持；

支持SAOP协议；

与MYSQL数据库服务器紧密结合，以对象方式支持MYSQL4.*的新特性

将新型数据库SQLLite绑定到PHP

流式控制加强

抛弃PHP4中的不合理部分

PHP5据说适用企业级开发，面向对象，类处理更加完善