PHP OOP - Final Classes and Methods

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:

<?php
// 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:

<?php
// 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.

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Daily Test with Code Example

HTML

CSS

JavaScript

PHP-MySQL

Which tag renders as emphasized text, displaying the text oblique?

<p>Web development courses: <em>CoursesWeb.net</em></p>

Which CSS property defines the space between the element border and its content?

margin padding position

h3 {
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}

Click on the method which returns the first element that matches a specified group of selectors.

getElementsByName() querySelector() querySelectorAll()

// gets first Div with class="cls", and shows its content
var elm = document.querySelector("div.cls");
alert(elm.innerHTML);

Indicate the PHP variable that contains data from a form sent with method="post".

$_SESSION $_GET $_POST

if(isset($_POST["field"])) {
  echo $_POST["field"];
}

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