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Class 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.

When inheriting from an abstract class, all methods marked abstract in the parent's class declaration must be defined by the child; additionally, these methods must be defined with the same (or a less restricted) visibility. For example, if the abstract method is defined as protected, the function implementation must be defined as either protected or public, but not private.

Example 10.18. Abstract class example

<?php
abstract class AbstractClass
{
   
// Force Extending class to define this method
   
abstract protected function getValue();
   abstract protected function
prefixValue($prefix);

   
// Common method
   
public function printOut() {
       print
$this->getValue() . "\n";
   }
}

class
ConcreteClass1 extends AbstractClass
{
   protected function
getValue() {
       return
"ConcreteClass1";
   }

   public function
prefixValue($prefix) {
       return
"{$prefix}ConcreteClass1";
   }
}

class
ConcreteClass2 extends AbstractClass
{
   public function
getValue() {
       return
"ConcreteClass2";
   }

   public function
prefixValue($prefix) {
       return
"{$prefix}ConcreteClass2";
   }
}

$class1 = new ConcreteClass1;
$class1->printOut();
echo
$class1->prefixValue('FOO_') ."\n";

$class2 = new ConcreteClass2;
$class2->printOut();
echo
$class2->prefixValue('FOO_') ."\n";
?>

The above example will output:

ConcreteClass1
FOO_ConcreteClass1
ConcreteClass2
FOO_ConcreteClass2


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