Final 关键字

PHP 5 新增了一个 final 关键字。如果父类中的方法被声明为 final，则子类无法覆盖该方法。如果一个类被声明为 final，则不能被继承。

Example #1 Final 方法示例

<?php
class BaseClass {
   public function test() {
       echo "BaseClass::test() called\n";
   }
   
   final public function moreTesting() {
       echo "BaseClass::moreTesting() called\n";
   }
}
class ChildClass extends BaseClass {
   public function moreTesting() {
       echo "ChildClass::moreTesting() called\n";
   }
}
// Results in Fatal error: Cannot override final method BaseClass::moreTesting()
?>

Example #2 Final 类示例

<?php
final class BaseClass {
   public function test() {
       echo "BaseClass::test() called\n";
   }
   
   // 这里无论你是否将方法声明为final，都没有关系
   final public function moreTesting() {
       echo "BaseClass::moreTesting() called\n";
   }
}
class ChildClass extends BaseClass {
}
// 产生 Fatal error: Class ChildClass may not inherit from final class (BaseClass)
?>

Note:属性不能被定义为 final，只有类和方法才能被定义为 final。

Note for Java developers: the 'final' keyword is not used for class constants in PHP. We use the keyword 'const'.
http://php.net/manual/en/language.oop5.constants.php

Note that you cannot ovverride final methods even if they are defined as private in parent class.
Thus, the following example:
<?php
class parentClass {
  final private function someMethod() { }
}
class childClass extends parentClass {
  private function someMethod() { }
}
?>
dies with error "Fatal error: Cannot override final method parentClass::someMethod() in ***.php on line 7"
Such behaviour looks slight unexpected because in child class we cannot know, which private methods exists in a parent class and vice versa.
So, remember that if you defined a private final method, you cannot place method with the same name in child class.

@thomas at somewhere dot com
The 'final' keyword is extremely useful. Inheritance is also useful, but can be abused and becomes problematic in large applications. If you ever come across a finalized class or method that you wish to extend, write a decorator instead.
<?php
final class Foo
{
  public method doFoo()
  {
    // do something useful and return a result
  }
}
final class FooDecorator
{
  private $foo;
  
  public function __construct(Foo $foo)
  {
    $this->foo = $foo;
  }
  
  public function doFoo()
  {
     $result = $this->foo->doFoo();
     // ... customize result ...
     return $result;
  }
}
?>

imo good to know:
<?php
class BaseClass
{
  protected static $var = 'i belong to BaseClass';
  public static function test()
  {
    echo '<hr>'.
      'i am `'.__METHOD__.'()` and this is my var: `'.self::$var.'`<br>';
  }
  public static function changeVar($val)
  {
    self::$var = $val;
    echo '<hr>'.
      'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>';
  }
  final public static function dontCopyMe($val)
  {
    self::$var = $val;
    echo '<hr>'.
      'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>';
  }
}
class ChildClass extends BaseClass
{
  protected static $var = 'i belong to ChildClass';
  public static function test()
  {
    echo '<hr>'.
      'i am `'.__METHOD__.'()` and this is my var: `'.self::$var.'`<br>'.
      'and this is my parent var: `'.parent::$var.'`';
  }
  public static function changeVar($val)
  {
    self::$var = $val;
    echo '<hr>'.
      'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>'.
      'but the parent $var is still: `'.parent::$var.'`';
  }
  public static function dontCopyMe($val) // Fatal error: Cannot override final method BaseClass::dontCopyMe() in ...
  {
    self::$var = $val;
    echo '<hr>'.
      'i am `'.__METHOD__.'()` and i just changed my $var to: `'.self::$var.'`<br>';
  }
}
BaseClass::test(); // i am `BaseClass::test()` and this is my var: `i belong to BaseClass`
ChildClass::test(); // i am `ChildClass::test()` and this is my var: `i belong to ChildClass`
          // and this is my parent var: `i belong to BaseClass`
ChildClass::changeVar('something new'); // i am `ChildClass::changeVar()` and i just changed my $var to: `something new`
                    // but the parent $var is still: `i belong to BaseClass`
BaseClass::changeVar('something different'); // i am `BaseClass::changeVar()` and i just changed my $var to: `something different`
BaseClass::dontCopyMe('a text'); // i am `BaseClass::dontCopyMe()` and i just changed my $var to: `a text`
ChildClass::dontCopyMe('a text'); // Fatal error: Cannot override final method BaseClass::dontCopyMe() in ...
?>

You can use final methods to replace class constants. The reason for this is you cannot unit test a class constant used in another class in isolation because you cannot mock a constant.  Final methods allow you to have the same functionality as a constant while keeping your code loosely coupled.
Tight coupling example (bad to use constants):
<?php
interface FooInterface
{
}
class Foo implements FooInterface
{
  const BAR = 1;
  public function __construct()
  {
  }
}
interface BazInterface
{
  public function getFooBar();
}
// This class cannot be unit tested in isolation because the actual class Foo must also be loaded to get the value of Foo::BAR
class Baz implements BazInterface
{
  private $foo;
  public function __construct(FooInterface $foo)
  {
    $this->foo = $foo;
  }
  public function getFooBar()
  {
    return Foo::BAR;
  }
}
$foo = new Foo();
$baz = new Baz($foo);
$bar = $baz->getFooBar();
?>
Loose coupling example (eliminated constant usage):
<?php
interface FooInterface
{
  public function bar();
}
class Foo implements FooInterface
{
  public function __construct()
  {
  }
  final public function bar()
  {
    return 1;
  }
}
interface BazInterface
{
  public function getFooBar();
}
// This class can be unit tested in isolation because class Foo does not need to be loaded by mocking FooInterface and calling the final bar method.
class Baz implements BazInterface
{
  private $foo;
  public function __construct(FooInterface $foo)
  {
    $this->foo = $foo;
  }
  public function getFooBar()
  {
    return $this->foo->bar();
  }
}
$foo = new Foo();
$baz = new Baz($foo);
$bar = $baz->getFooBar();
?>

The behaviour of FINAL is not as serious as you may think. A little explample:
<?php
class A {
   final private function method(){}   
}
class B extends A {
  private function method(){}
}
?>
Normally you would expect some of the following will happen:
- An error that final and private keyword cannot be used together
- No error as the private visibility says, that a method/var/etc. is only visible within the same class
But what happens is PHP is a little curios: "Cannot override final method A::method()"
So its possible to deny method names in subclasses! Don't know if this is a good behavior, but maybe its useful for your purpose.

The use of final keyword is just like that occurs in Java
In java final has three uses
  1) prevent class Inheritance
  2) prevent method overriding or redifination of 
    method in subclass 
  3) and to declare constants 
 But the third point seems to be missing from the PHP
 I guess, as i am a java developer Currently gaining competence in PHP

<?php
class parentClass {
  public function someMethod() { }
}
class childClass extends parentClass {
  public final function someMethod() { } //override parent function
}
$class = new childClass;
$class->someMethod(); //call the override function in chield class
?>

"Note for Java developers: the 'final' keyword is not used for class constants in PHP. We use the keyword 'const'."
http://php.net/manual/en/language.oop5.constants.php
This is more or less true, regardless of the fact that constant (being defined at class level or not) in PHP are only scalar (int, string, etc) while in Java they may be pure object (ex: java.awat.Color.BLACK). The only possible solution of having such kind of constant is :
<?php
class Bar {...}
class Foo {
 public static $FOOBAR;
 static function __init() {
  static $init = false;
  if ($init) throw new Exception('Constants were already initialized');
  self::$FOOBAR = new Bar();
  $init = true;
 }
}
Foo::__init();
?>
That said, perhaps it is useless unless PHP automatically calls the __init() method.
However, one alternative that could be done in certain case is this : 
<?php
function __autoload($className) { 
 ... require the file where the class is ...
 if (interface_exists($className, false)) return;
 if (class_exists($className, false)) {
  $rc = new ReflectionClass($className); 
  if (!$rc->hasMethod('__init')) return;
  $m = $rc->getMethod('__init');
  if (!($m->isStatic() && $m->isPrivate())) {
   throw new Exception($className . ' __init() method must be private and static !');
  }
  $m->invoke(null);
  return;
 }
 throw new Exception('Class or interface not found ' . $className);
}
?>
This can only work when one class is defined per file, since we are assured that __autoload() will be called to load the file containing the class.
eg: 
test2.php:
<?php
class B {
 public static $X;
 private static function __init() {
  echo 'B', "\n"; 
  self::$X = array(1, 2);
 }
}
class A {
 public static $Y;
 private static function __init() {
  echo 'A', "\n"; 
  self::$Y = array(3, 4);
 }
}
?>
test.php:
<?php
function __autoload($n) {
 if ($n == 'A' || $n == 'B') require 'test2.php';
 ... do our __init() trick ...
}
var_dump(B::$X); // shows B, then array(2) (1, 2)
var_dump(A::$Y); // shows NULL.
?>

Final is not really about overriding or overloading a method.
PHP doesn't support (yet ?) method ovverriding.
The final keyword however prevent you to redefine a method already previously declared.

@someone
@thomas
Decorating a finalized class is not possible. The decorator that's mentioned is incomplete. There's a fundamental flaw in it. Look:
<?php
//copy'n'paste your FooBar, Foo definition
$f = new Foo;
$fd = new FooDecorator($f);
var_dump($fd instanceof $f); //FALSE
var_dump(is_a($fd, 'Foo')); //FALSE
fooFoo($fd); //E_RECOVERABLE_ERROR here !!!;
?>
What you've created is just an object that happens to have the same methods(a duck type). But if in the client code someone makes decision based on the type of your passed decorator they'll make incorrect decision - or to be more precise not the one that you, the author of the 'decorator' wants them to make. 
FYI that's the correct implementation based on GoF:
<?php
class Foo
{
  public method doFoo()
  {
    // do something useful and return a result
  }
}
//decorator must inherit the interface(the methods, type info etc.) //of the decorated class by *extending* it.
class FooDecorator extends Foo 
{
  private $foo;
  
  public function __construct(Foo $foo)
  {
    $this->foo = $foo;
  }
  
  public function doFoo()
  {
     $result = $this->foo->doFoo();
     // ... customize result ...
     return $result;
  }
}
function fooFoo(Foo $f) {}
$f = new Foo;
$fd = new FooDecorator($f);
var_dump($fd instanceof $f); //true
fooFoo($fd); //no E_RECOVERABLE_ERROR here;
?>
I haven't come across any legitimate use of a finalizing class/method and I personally think that 'final' is has no much use and is just a copy'n'pasted from Java into PHP. The keyword makes code difficult to test. If you have to create a test double from a finalized class because you'll need create a derived type to shadow the methods you don't care about. If one of them is finalized you've already lost.

Right way:
 final protected function example() {
}
Wrong way:
protected final function example() {
}
Source: Practices

Note: Properties cannot be declared final, only classes and methods may be declared as final.
you can use trait:
<?php
trait PropertiesTrait {
  public $same = true;
  public $different = false;
}
class PropertiesExample {
  use PropertiesTrait;
  public $same = true; // Strict Standards
  public $different = true; // 致命错误
}
?>