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Notes An object is an instance of a class. In this case, it is an actual user data structure with attached
code. Objects and classes are a bit like values and data types. There’s only one integer data type,
but there are many possible integers. Similarly, your program defines only one user class but
can create many different (or identical) users from it.
The data associated with an object are called its properties. The functions associated with an
object are called its methods. When you define a class, you define the names of its properties
and give the code for its methods.

Debugging and maintenance of programs is much easier if you use encapsulation. This is the
idea that a class provides certain methods (the interface) to the code that uses its objects, so the
outside code does not directly access the data structures of those objects. Debugging is thus
easier because you know where to look for bugs. The only code that changes an object’s data
structures is in the class—and maintenance is easier because you can swap out implementations of
a class without changing the code that uses the class, as long as you maintain the same interface.
Any nontrivial object-oriented design probably involves inheritance. This is a way of defining a
new class by saying that it is like an existing class, but with certain new or changed properties
and methods. The old class is called the super class (or base class), and the new class is called
the subclass (or derived class). Inheritance is a form of code reuse—the base-class code is reused
instead of being copied and pasted into the new class. Any improvements or modifications to
the base class are automatically passed on to the derived class.

8.3 Creating an Object

It is much easier to create objects and use them than it is to define object classes, so before we
discuss how to define classes, let’s look at creating objects. To create an object of a given class,
use the new keyword:

$object = new Class;
Assuming that a Person class has been defined, here’s how to create a Person object:
$rasmus = new Person;

Do not quote the class name, or you will get a compilation error:
$rasmus = new ‘Person’; // does not work
Some classes permit you to pass arguments to the new call. The class’s documentation should
say whether it accepts arguments. If it does, you will create objects like this:
$object = new Person(‘Fred’, 35);

The class name does not have to be hardcoded into your program. You can supply the class
name through a variable:

$class = ‘Person’; $object = new $class; // is equivalent to $object = new Person;
Specifying a class that does not exist causes a runtime error.
Variables containing object references are just normal variables—they can be used in the same
ways as other variables. Of particular note is that variable variables work with objects, as shown
here:
$account = new Account; $object = ‘account’ ${$object}->init(50000, 1.10); // same as $account-
>init

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