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.

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
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 = "http://CoursesWeb.net/";
$nr_chr = strlen($str);
echo $nr_chr;       // 22
PHP OOP - Final Classes and Methods

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