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<A PHP V5 migration guide ⁄ >

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With the new language features of PHP V5, you can significantly improve your code's maintainability and stability. Learn how to migrate code developed in PHP V4 to V5 while taking advantage of these new features.

PHP V5 is a quantum step up from V4. The new language features make building reliable and maintaining class libraries much easier. In addition, the reworking of the standard libraries helped bring PHP more in line with its cousin Web languages, such as the Java™ programming language. Take a tour through some of PHP's new object-oriented features and learn how to migrate existing PHP V4 code into V5.

First, a bit about the new language features and how PHP's creators have changed the approach to objects from PHP V4. The idea with V5 was to create an industrial-strength language for Web application development. That meant understanding the limitations of PHP V4, then pulling known good language constructs from other languages (such as the Java, C#, C++, Ruby, and Perl languages) and incorporating them into PHP.

The first and most important addition was access protection for methods and instance variables on classes -- the public, protected, and private keywords. This addition allows class designers to retain control over the internals of their classes while expressing to the client of the class what he should or should not touch.

In PHP V4, everything was public. In PHP V5, class designers can say what is externally visible (public) and what is visible only internally to the class (private) or to descendants of the class (protected). Without these access controls, working on code in large groups or distributing code as libraries was hindered because consumers of those classes could easily use the wrong methods or access what should have been private member variables.

Another big addition were the keywords interface and abstract, which allow for contract programming. Contract programming means that one class provides a contract to another -- in other words: "Here is what I will do, and you don't need to know how it's done." Any class that implements that interface agrees to the contract. Any consumer of the interface agrees to use only the methods specified in the interface. The abstract keyword makes using interfaces a lot easier, as I show later.

These primary two features -- access control and contract programming -- allow for much larger teams of coders to work on much larger code bases more smoothly. They also allow IDEs to provide a much richer set of language intelligence features. Although this article covers several migration issues, I also spend a lot of time showing how to use these new key language features.

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