Final classes

Inheritance allows for enormous flexibility within a class hierarchy. You can create subclasses to extend the functionality of a base class, but PHP OOP gives the posibility to create classes that cannot be extended. Such a class is called final class.
A final class is declared by adding the final keyword before the class word.
  - For example:
// base class (declared final)
final class Base {
  // ... PHP code

// define a child class derivated from Base
class BaseChild extends Base {
  // ... PHP instructions
Running this code will generate the following error:
Fatal error: Class BaseChild may not inherit from final class (Base) in ...

Final methods

A normal method (public or protected) can be overridden in the child class. If you want a method to remain fixed and unchanging, prefix the definition with the final keyword.
A final method cannot be overridden.
  - For example:
// base class
class Base {
  // final method
  final public function testMethod() {
    echo 'This is a final method';

// define a child class derivated from Base
class BaseChild extends Base {
  // override the testMethod()
  public function testMethod() {
    echo 'Another text';
This example will generate the following error:
Fatal error: Cannot override final method Base::testMethod() in ...

By declaring a class or method final, you limit the flexibility given by inheritance, so, use final with care.

Daily Test with Code Example

Which tag is used to include external CSS file in web page?
<body> <script> <link>
<link href="/templ/style.css" rel="stylesheet" type="text/css" />
Which CSS property sets the text size?
font-weight text-decoration font-size
h2 {
  font-size: 1em;
Indicate the JavaScript property that can add HTML code into an element.
text value innerHTML
document.getElementById("someID").innerHTML = "HTML content";
Click on the function that returns the number of characters of a string in PHP.
count() strlen() stristr()
$str = "";
$nr_chr = strlen($str);
echo $nr_chr;       // 22
PHP OOP - Final Classes and Methods

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