PHP

OOP – Abstract Classes

PHP – What are Abstract Classes and Methods?

Abstract classes and methods are when the parent class has a named method,
but
need its child class(es) to fill out the tasks.

An abstract class is a class that contains at least one abstract method. An
abstract method is a method that is declared, but not implemented in the code.

An abstract class or method is defined with the abstract
keyword:

Syntax

<?php
abstract class
ParentClass {
  abstract public function someMethod1();
 
abstract public function someMethod2($name, $color);
  abstract
public function someMethod3() : string;
}
?>

When inheriting from an abstract class, the child class method must be
defined with the same name, and the same or a less restricted access modifier.
So, if the abstract method is defined as protected, the child class method must
be defined as either protected or public, but not private. Also, the type and number of
required arguments must be the same. However, the child classes may have optional
arguments in addition.

So, when a child class is inherited from an abstract class, we have the following rules:

• The child class method must be defined with the same name and it redeclares the parent abstract method
• The child class method must be defined with the same or a less restricted
  access modifier
• The number of required arguments must be the same. However, the child class
  may have optional
  arguments in addition

Let’s look at an example:

Example

<?php
// Parent class
abstract class Car {
  public
$name;
  public
function __construct($name) {
    $this->name = $name;
  }
 
abstract public function intro() : string;
}

// Child classes
class
Audi extends Car {
  public
function intro() : string {
    return “Choose German
quality! I’m an $this->name!”;

  }
}

class
Volvo extends Car {
  public
function intro() : string {
    return “Proud to be
Swedish! I’m a $this->name!”;

  }
}

class
Citroen extends Car {
  public
function intro() : string {
    return “French
extravagance! I’m a $this->name!”;

  }
}

// Create objects from the child classes
$audi = new
audi(“Audi”);
echo $audi->intro();
echo “<br>”;

$volvo = new
volvo(“Volvo”);
echo $volvo->intro();
echo “<br>”;

$citroen = new citroen(“Citroen”);
echo $citroen->intro();
?>

Try it Yourself »

Example Explained

The Audi, Volvo, and Citroen classes are inherited from the Car class. This means that the
Audi, Volvo, and Citroen classes can use the public $name
property as well as the public __construct() method from the
Car class because of inheritance.

But, intro() is an abstract method that should be defined in all the child
classes and they should return a string.

PHP – More Abstract Class Examples

Let’s look at another example where the abstract method has an argument:

Example

<?php
abstract class ParentClass {
  // Abstract method
with an argument
  abstract protected
function prefixName($name);
}

class ChildClass extends ParentClass {
  public function prefixName($name) {
   
if ($name == “John Doe”) {
      $prefix = “Mr.”;
   
} elseif ($name == “Jane Doe”) {
      $prefix =
“Mrs.”;
    } else {
     
$prefix = “”;
    }
    return
“{$prefix} {$name}”;
  }
}

$class = new ChildClass;
echo $class->prefixName(“John
Doe”);
echo “<br>”;
echo $class->prefixName(“Jane Doe”);
?>

Try it Yourself »

Let’s look at another example where the abstract method has an argument, and
the child class has two optional arguments that are not defined in the parent’s
abstract method:

Example

<?php
abstract class ParentClass {
  // Abstract method
with an argument
  abstract protected
function prefixName($name);
}

class ChildClass extends ParentClass {
 
// The child class may define optional arguments that are not in the parent’s
abstract method
  public function prefixName($name, $separator = “.”,
$greet = “Dear”) {
   
if ($name == “John Doe”) {
      $prefix = “Mr”;
   
} elseif ($name == “Jane Doe”) {
      $prefix =
“Mrs”;
    } else {
     
$prefix = “”;
    }
    return
“{$greet} {$prefix}{$separator} {$name}”;
  }
}

$class = new ChildClass;
echo $class->prefixName(“John
Doe”);
echo “<br>”;
echo $class->prefixName(“Jane Doe”);
?>

Try it Yourself »