What is an abstract class in PHP?

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.

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());

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.