Constructors are functions in a class that are automatically called when you create a new instance of a class with new. A function becomes a constructor, when it has the same name as the class. If a class has no constructor, the constructor of the base class is being called, if it exists.
This defines a class Auto_Cart that is a Cart plus a constructor which initializes the cart with one item of article number "10" each time a new Auto_Cart is being made with "new". Constructors can take arguments and these arguments can be optional, which makes them much more useful. To be able to still use the class without parameters, all parameters to constructors should be made optional by providing default values.
You also can use the @ operator to mute errors occurring in the constructor, e.g. @new.
The function B() in class A will suddenly become a constructor in class B, although it was never intended to be. PHP 4 does not care if the function is being defined in class B, or if it has been inherited.
PHP 4 doesn't call constructors of the base class automatically from a constructor of a derived class. It is your responsibility to propagate the call to constructors upstream where appropriate.
Destructors are functions that are called automatically when an object is destroyed, either with unset() or by simply going out of scope. There are no destructors in PHP. You may use register_shutdown_function() instead to simulate most effects of destructors.