PHP 手冊
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# Object Abstraction

PHP 5 introduces abstract classes and methods. It is not allowed to create an instance of a class that has been defined as abstract. Any class that contains at least one abstract method must also be abstract. Methods defined as abstract simply declare the method's signature they cannot define the implementation.

The class that implements the abstract method must define with the same visibillity or weaker. If the abstract method is defined as protected, the function implementation must be defined as either protected or public.

範例 19-16. Abstract class example

`<?php
abstract class AbstractClass
{
// Force Extending class to define this method
abstract protected function getValue();
// Common method
public function printOut() {
print $this->getValue();
}
}
class ConcreteClass1 extends AbstractClass
{
protected function getValue() {
return "ConcreteClass1";
}
}
class ConcreteClass2 extends AbstractClass
{
protected function getValue() {
return "ConcreteClass2";
}
}
$class1 = new ConcreteClass1;
$class1->printOut();
$class2 = new ConcreteClass2;
$class2->printOut();
?>`

Old code that has no user-defined classes or functions named 'abstract' should run without modifications.

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