What is an abstract class in PHP?

Question

What is an abstract class in PHP?

How can it be used?

Adductor

(reference) de2.php.net/manual/en/language.oop5.abstract.php – Completion

(related) #2288967 – Completion

@Gordon: Sometimes official docs are not that clear for some people :( – Birl

(duplicate) https://mcmap.net/q/47941/-abstract-keyword-in-php – Completion

@Birl that isn't meant in an RTFM way. I just think that having a link to the official documentation is a helpful asset. And you wouldn't believe how many people don't read official docs or even know they exist. – Completion

Answer 1

An abstract class is a class that contains at least one abstract method, which is a method without any actual code in it, just the name and the parameters, and that has been marked as "abstract".

The purpose of this is to provide a kind of template to inherit from and to force the inheriting class to implement the abstract methods.

An abstract class thus is something between a regular class and a pure interface. Also interfaces are a special case of abstract classes where ALL methods are abstract.

See this section of the PHP manual for further reference.

Cyclopentane

In this thread, the abstract does not contain any abstract method. – Outcrop

@r0ng It would be more correct to say that an abstract class may contain abstract methods, and abstract methods must always be inside an abstract class. The defining practical features in my mind are that an abstract class can not be instantiated, but you can inherit and overwrite methods, and static methods from an abstract class can be used normally. – Antiar

Answer 2

Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.

1. Can not instantiate abstract class: Classes defined as abstract may not be instantiated, and any class that contains at least one abstract method must also be abstract.

Example below :

abstract class AbstractClass
{

    abstract protected function getValue();
    abstract protected function prefixValue($prefix);


    public function printOut() {
        echo "Hello how are you?";
    }
}

$obj=new AbstractClass();
$obj->printOut();
//Fatal error: Cannot instantiate abstract class AbstractClass

2. Any class that contains at least one abstract method must also be abstract: Abstract class can have abstract and non-abstract methods, but it must contain at least one abstract method. If a class has at least one abstract method, then the class must be declared abstract.

Note: Traits support the use of abstract methods in order to impose requirements upon the exhibiting class.

Example below :

class Non_Abstract_Class
{
   abstract protected function getValue();

    public function printOut() {
        echo "Hello how are you?";
    }
}

$obj=new Non_Abstract_Class();
$obj->printOut();
//Fatal error: Class Non_Abstract_Class contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Non_Abstract_Class::getValue)

3. An abstract method can not contain body: Methods defined as abstract simply declare the method's signature - they cannot define the implementation. But a non-abstract method can define the implementation.

abstract class AbstractClass
{
   abstract protected function getValue(){
   return "Hello how are you?";
   }

    public function printOut() {
        echo $this->getValue() . "\n";
    }
}

class ConcreteClass1 extends AbstractClass
{
    protected function getValue() {
        return "ConcreteClass1";
    }

    public function prefixValue($prefix) {
        return "{$prefix}ConcreteClass1";
    }
}

$class1 = new ConcreteClass1;
$class1->printOut();
echo $class1->prefixValue('FOO_') ."\n";
//Fatal error: Abstract function AbstractClass::getValue() cannot contain body

4. When inheriting from an abstract class, all methods marked abstract in the parent's class declaration must be defined by the child :If you inherit an abstract class you have to provide implementations to all the abstract methods in it.

abstract class AbstractClass
{
    // Force Extending class to define this method
    abstract protected function getValue();

    // Common method
    public function printOut() {
        print $this->getValue() . "<br/>";
    }
}

class ConcreteClass1 extends AbstractClass
{
    public function printOut() {
        echo "dhairya";
    }

}
$class1 = new ConcreteClass1;
$class1->printOut();
//Fatal error: Class ConcreteClass1 contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (AbstractClass::getValue)

5. Same (or a less restricted) visibility:When inheriting from an abstract class, all methods marked abstract in the parent's class declaration must be defined by the child; additionally, these methods must be defined with the same (or a less restricted) visibility. For example, if the abstract method is defined as protected, the function implementation must be defined as either protected or public, but not private.

Note that abstract method should not be private.

abstract class AbstractClass
{

    abstract public function getValue();
    abstract protected function prefixValue($prefix);

        public function printOut() {
        print $this->getValue();
    }
}

class ConcreteClass1 extends AbstractClass
{
    protected function getValue() {
        return "ConcreteClass1";
    }

    public function prefixValue($prefix) {
        return "{$prefix}ConcreteClass1";
    }
}
$class1 = new ConcreteClass1;
$class1->printOut();
echo $class1->prefixValue('FOO_') ."<br/>";
//Fatal error: Access level to ConcreteClass1::getValue() must be public (as in class AbstractClass)

6. Signatures of the abstract methods must match:When inheriting from an abstract class, all methods marked abstract in the parent's class declaration must be defined by the child;the signatures of the methods must match, i.e. the type hints and the number of required arguments must be the same. For example, if the child class defines an optional argument, where the abstract method's signature does not, there is no conflict in the signature.

abstract class AbstractClass
{

    abstract protected function prefixName($name);

}

class ConcreteClass extends AbstractClass
{


    public function prefixName($name, $separator = ".") {
        if ($name == "Pacman") {
            $prefix = "Mr";
        } elseif ($name == "Pacwoman") {
            $prefix = "Mrs";
        } else {
            $prefix = "";
        }
        return "{$prefix}{$separator} {$name}";
    }
}

$class = new ConcreteClass;
echo $class->prefixName("Pacman"), "<br/>";
echo $class->prefixName("Pacwoman"), "<br/>";
//output: Mr. Pacman
//        Mrs. Pacwoman

7. Abstract class doesn't support multiple inheritance:Abstract class can extends another abstract class,Abstract class can provide the implementation of interface.But it doesn't support multiple inheritance.

interface MyInterface{
    public function foo();
    public function bar();
}

abstract class MyAbstract1{
    abstract public function baz();
}


abstract class MyAbstract2 extends MyAbstract1 implements MyInterface{
    public function foo(){ echo "foo"; }
    public function bar(){ echo "bar"; }
    public function baz(){ echo "baz"; }
}

class MyClass extends MyAbstract2{
}

$obj=new MyClass;
$obj->foo();
$obj->bar();
$obj->baz();
//output: foobarbaz

Note: Please note order or positioning of the classes in your code can affect the interpreter and can cause a Fatal error. So, when using multiple levels of abstraction, be careful of the positioning of the classes within the source code.

below example will cause Fatal error: Class 'horse' not found

class cart extends horse {
    public function get_breed() { return "Wood"; }
}

abstract class horse extends animal {
    public function get_breed() { return "Jersey"; }
}

abstract class animal {
    public abstract function get_breed();
}

$cart = new cart();
print($cart->get_breed());

Rhomboid

if we will take step 6 and just write first letter in uppercase for the function name in child class. The code will run correctly - why? ... class ConcreteClass extends AbstractClass { public function PrefixName($name, $separator = ".") { ... } ... – Perilune

Answer 3

An abstract class is a class that is only partially implemented by the programmer. It may contain one or more abstract methods. An abstract method is simply a function definition that serves to tell the programmer that the method must be implemented in a child class.

There is good explanation of that here.

Birl

Hi, thanks for the explanation, the truth is that there is no such good explanation as this on the internet about abstract and interface in PHP. – Boorman

Answer 4

Abstract Class
1. Contains an abstract method
2. Cannot be directly initialized
3. Cannot create an object of abstract class
4. Only used for inheritance purposes

Abstract Method
1. Cannot contain a body
2. Cannot be defined as private
3. Child classes must define the methods declared in abstract class

Example Code:

abstract class A {
    public function test1() {
        echo 'Hello World';
    }
    abstract protected function f1();
    abstract public function f2();
    protected function test2(){
        echo 'Hello World test';
    }
}

class B extends A {
    public $a = 'India';
    public function f1() {
        echo "F1 Method Call";
    }
    public function f2() {
        echo "F2 Method Call";
    }
}

$b = new B();
echo $b->test1() . "<br/>";
echo $b->a . "<br/>";
echo $b->test2() . "<br/>";
echo $b->f1() . "<br/>";
echo $b->f2() . "<br/>";

Output:

Hello World
India
Hello World test
F1 Method Call
F2 Method Call

Cardona

you can't call the parent protected method by child class object , you can call in child class method by this object echo $b->test2() . "<br/>"; – Guillotine

Answer 5

• Abstract Class contains only declare the method's signature, they can't define the implementation.
• Abstraction class are defined using the keyword abstract .
• Abstract Class is not possible to implement multiple inheritance.
• Latest version of PHP 5 has introduces abstract classes and methods.
• Classes defined as abstract , we are unable to create the object ( may not instantiated )

Procter

Answer 6

An abstract class is like the normal class it contains variables it contains protected variables functions it contains constructor only one thing is different it contains abstract method.

The abstract method means an empty method without definition so only one difference in abstract class we can not create an object of abstract class

Abstract must contains the abstract method and those methods must be defined in its inheriting class.

Jacklighter