Class implementing an interface must be declared before a children class?

Question

I stumbled on an interesting issue today. In the context of a single file for multiple declarations, if class B implements interface A, and class C extends class A, class B must be declared before class C.

The following code does not work:

interface A {}
class C extends B {} // Class 'B' not found
class B implements A {}

This fixes it:

interface A {}
class B implements A {}
class C extends B {} // Class 'B' is found

But this works fine:

class A {}
class C extends B {} // Class 'B' is found
class B extends A {}

These are my results on PHP 5.3.8 (Win32) and PHP 5.3.3-1ubuntu9.6 w/ suhosin.

So the question is, is this documented behavior? Why would it work with classes but not interfaces? Or should this be considered a bug?

Let me know about your experience, before I go digging in the PHP's source code and/or open a PHP bug ticket. :)

Thanks!

Note: I know it's just a matter of class declaration order, but this puzzles me... If it's inappropriate, please don't hesitate to close.

Answer 1

Yes, this is documented behavior:

Object Inheritance (Note):

Unless autoloading is used, then classes must be defined before they are used. If a class extends another, then the parent class must be declared before the child class structure. This rule applies to class that inherit other classes and interfaces.

 

PHP: extends - Manual (Note)

Classes must be defined before they are used! If you want the class Named_Cart to extend the class Cart, you will have to define the class Cart first. If you want to create another class called Yellow_named_cart based on the class Named_Cart you have to define Named_Cart first. To make it short: the order in which the classes are defined is important.

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Undefined Behavior (with added sarcasm)

Even though your third example might word, it's not said to work according to the documentation (and I cannot find any changelog entry stating that it's a feature brought in).

The documentation of PHP sadly isn't 100%, but you can only trust what you know, and the above quotes are all I can find regarding this matter.

You might call it a bug, but in C++ we would describe it as undefined behavior ie. something that shouldn't be relied on.

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If the "standard" (documentation) doesn't mention it, it's not safe to use.

If the "standard" (documentation) doesn't mention it, it may make the universe implode - or a raptor jump through your window. Do not do anything that can kill us all!