Here's a quick question for the wider PHP programming community '¦ if you're writing code that tests for the presence of an array, should is_array() also accept objects that behave like arrays?

Some quick code snippets show that, currently in PHP 5.2, an is_array test will return false. If you use an instanceof to check it, however, you can get it to return true. There's plenty of comments on the subject with quite a few "no"s in the group.

This is a fantastic way to further encapsulate and abstract your code because it means you can define some base functionality and then later on extend that class to add new functionality and even override existing functionality to make the class specific. But this concept is a double-edged sword in PHP (and all other languages).

The problem lies in methods in classes that could be overriding parents and the sort of results that instance of might return in a multiple inheritance environment. He includes code snippets both illustrating the problem and showing a suggestion for how it could be avoided.

Many OO languages have the concept of a single base class from which all other classes are explicitly or implicitly descended. For example, Ruby, Java and .NET all have Object. It's a very common belief that PHP implements stdClass as a base class for all objects, but this is in fact not the case.

He illustrates with a code example showing the results of calls to instanceof with a normal user class and one that extends the stdClass object.

Well, if you are into duck typing style of programming, it may be interesting for you to have an object that implements certain set of functions, but not necessary declares it at class definition. Languages like Smalltalk do it all day along, so why PHP couldn't?

His example defines an interface Cow and a class MooingGrassEater and a function, CowConsumer, that does the work. A classname is passed in and an instance of that class is checked with "implements" rather than "instanceof" to see if it uses the Cow interface. He points out a place where PHP itself uses something similar in user defined streams.

This three-part series goes through the basic concepts of object type enforcement in PHP 4/PHP 5. It explores different approaches for checking types of objects to help you avoid possible code contamination when objects of incorrect type are inputted within PHP classes.

The end result of this article hopefully will help you to expand your grounding in how to implement object type enforcement in PHP 5, by developing some illustrative, object-oriented examples.

They start with a look at how not to do the object checking but provide a solution to the situation with the help of the instanceof operator. They explain its usage in the context of the widget class they've created, and show you how it can be integrated easily to simplify object validation.]]> Thu, 23 Feb 2006 07:01:43 -0600