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 is used to add lists into <ul> and <ol> elements?

<ul>
 <li>http://coursesweb.net/html/</li>
 <li>http://coursesweb.net/css/</li>
</ul>

Which value of the "display" property creates a block box for the content and ads a bullet marker?

block list-item inline-block

.some_class {
  display: list-item;
}

Which instruction converts a JavaScript object into a JSON string.

JSON.parse() JSON.stringify eval()

var obj = {
 "courses": ["php", "javascript", "ajax"]
};
var jsonstr = JSON.stringify(obj);
alert(jsonstr);    // {"courses":["php","javascript","ajax"]}

Indicate the PHP class used to work with HTML and XML content in PHP.

stdClass PDO DOMDocument

$strhtml = '<body><div id="dv1">CoursesWeb.net</div></body>';
$dochtml = new DOMDocument();
$dochtml->loadHTML($strhtml);
$elm = $dochtml->getElementById("dv1");
echo $elm->nodeValue;    // CoursesWeb.net

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