In Java, a Class can Implement multiple Interfaces.
Is the same true for PHP5?
TomTees
Yes.
class DoubleLinkedList implements Cloneable, Serializiable {
// etc..
}
Okay, thanks guys!
TomTees
In PHP a class can only extend one other class though, not multiple (sadly). If you need to extend several classes you can do it like so:
class A {
}
class B extends class A {
}
class C extends class B {
}
// etc ...
That way class C extends both class A and B. This can get messy real quick though; use with care 
Can you Extends multiple classes in other languages like Java and C++?
Is it bad design to want to extend a class multiple times anyways?
TomTees
Java: no, C++: yes
As for bad design, well that depends on what you want to do. The principle in itself is not bad, but as with a lot of things it can be used wrongly.
As a general rule of thumb I’d say single-level inheritance is OK, two-level inheritance is tolerable, three-level inheritance is very questionable, and if you want to go even further than that you should probably go back to the drawing board
Last I heard mixins are planned for PHP 6. So when 6 comes out multiple inheritance will be possible using mixins.
More precisely, traits have been committed to trunk. That may end up becoming 5.4 or 6. I’m guessing the former.
Traits are a form of mixins, but they are resolved at static time. I think this matches better with php’s class model.
See this page for a detailed discussion: http://wiki.php.net/rfc/horizontalreuse
Cerium’s example is correct; and all classes must explicitly be defined as interface, to implement it.
<?php
interface Cloneable
{
}
interface Serializiable
{
}
class DoubleLinkedList implements Cloneable, Serializiable {
}
$dll = new DoubleLinkedList();
?>
Many MVC’s frameworks do not support interfaces either. So if you plan on making something and then porting it over to an MVC framework be careful. Kohana 3 does allow the use of interfaces.
Most others only support abstract classes, because at the core many MVC frameworks still support php 4.
You mean faux abstract classes, PHP 4 doesn’t have the “abstract” keyword or enforcement.