PHP4 included the first release of the Zend engine version 1.0, PHP’s scripting engine that implements the syntax of the language and provides all of the tools needed to run library functions. PHP5 includes a new Zend engine version 2.0, that’s enhanced to address the limitations of version 1.0 and to include new features that have been requested by developers. However, unlike the changes that occurred when PHP3 became PHP4, the changes from PHP4 to PHP5 only affect part of the language. Most code that’s written for PHP4 will run without modification under PHP5.

In brief, the following are the major new features in PHP5. Many of these features are explained in detail elsewhere in this book:

New Object Model

Object-oriented programming (OOP) and the OOP features of PHP5 are discussed in detail in Chapter 14. PHP4 has a simple object model that doesn’t include many of the features that object-oriented programmers expect in an OOP language such as destructors, private and protected member functions and variables, static member functions and variables, interfaces, and class type hints. All of these features are available in PHP5.

The PHP5 OOP model also better manages how objects are passed around between functions and classes. Handles to objects are now passed, rather than the objects themselves. This has substantially improved the performance of PHP.

Internationalization

Support for non-Western character sets and Unicode. This is discussed in Chapter 3.

Exception Handling

New try...catch , and throw statements are available that are aimed at improving the robustness of applications when errors occur. These are discussed in Chapter 4. There’s also a backtrace feature that you can use to develop a custom error handler that shows how the code that caused an error was called. This feature has been back-ported into PHP4 and is discussed in Chapter 12.

Improved memory handling and speed

PHP4 was fast, but PHP5 is faster and makes even better use of memory. We don’t discuss this in detail.

New XML support

There were several different tools for working with the eXtensible Markup Language (XML) in PHP4. These tools have been replaced with a single new, robust framework in PHP5. We don’t discuss XML support in this book.

The Improved MySQL library (mysqli)

A new MySQL function library is available in PHP5 that supports MySQL 4. The library has the significant feature that it allows an SQL query to be prepared once, and executed many times, and this substantially improves speed if a query is often used. This library is briefly described in Chapter 6, and is the source of many of the PHP4 and PHP5 compatibility problems described throughout in this book.

You can find out more about what’s new in PHP5 from http://www.zend.com/zend/future.php.