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<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.9 $ -->
<sect1 id="language.oop5.abstract">
<title>Object Abstraction</title>
<para>
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.
</para>
<para>
The class that implements the abstract method must define with the same
<link linkend="language.oop5.visibility">visibillity</link> or weaker. If the
abstract method is defined as protected, the function implementation must be
defined as either protected or public.
</para>
<example>
<title>Abstract class example</title>
<programlisting role="php">
<![CDATA[
<?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
{
public function getValue() {
return "ConcreteClass2";
}
}
$class1 = new ConcreteClass1;
$class1->printOut();
$class2 = new ConcreteClass2;
$class2->printOut();
?>
]]>
</programlisting>
</example>
<para>
Old code that has no user-defined classes or functions named
'abstract' should run without modifications.
</para>
</sect1>
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