PHP 7.0.6 Released

    Class Abstraction

    PHP 5 introduces abstract classes and methods. Classes defined as abstract may not be instantiated, and any class that contains at least one abstract method must also be abstract. Methods defined as abstract simply declare the method's signature - they cannot define the implementation.

    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. Furthermore 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. This also applies to constructors as of PHP 5.4. Before 5.4 constructor signatures could differ.

    Example #1 Abstract class example

    <?php
    abstract class AbstractClass
    {
            // Force Extending class to define this method
            abstract protected function getValue();
        abstract protected function     prefixValue($prefix);

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

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

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

    class     ConcreteClass2 extends AbstractClass
    {
        public function     getValue() {
            return     "ConcreteClass2";
        }

        public function     prefixValue($prefix) {
            return     "{$prefix}ConcreteClass2";
        }
    }

    $class1 = new ConcreteClass1;
    $class1->printOut();
    echo     $class1->prefixValue('FOO_') ."\n";

    $class2 = new ConcreteClass2;
    $class2->printOut();
    echo     $class2->prefixValue('FOO_') ."\n";
    ?>

    The above example will output:

    ConcreteClass1
FOO_ConcreteClass1
ConcreteClass2
FOO_ConcreteClass2

    Example #2 Abstract class example

    <?php
    abstract class AbstractClass
    {
            // Our abstract method only needs to define the required arguments
            abstract protected function prefixName($name);

    }

    class     ConcreteClass extends AbstractClass
    {

            // Our child class may define optional arguments not in the parent's signature
            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"), "\n";
    echo     $class->prefixName("Pacwoman"), "\n";
    ?>

    The above example will output:

    Mr. Pacman
Mrs. Pacwoman

    Old code that has no user-defined classes or functions named 'abstract' should run without modifications.

    User Contributed Notes

    ironiridis at gmail dot com
    8 years ago
    Just one more time, in the simplest terms possible:

    An Interface is like a protocol. It doesn't designate the behavior of the object; it designates how your code tells that object to act. An interface would be like the English Language: defining an interface defines how your code communicates with any object implementing that interface.

    An interface is always an agreement or a promise. When a class says "I implement interface Y", it is saying "I promise to have the same public methods that any object with interface Y has".

    On the other hand, an Abstract Class is like a partially built class. It is much like a document with blanks to fill in. It might be using English, but that isn't as important as the fact that some of the document is already written.

    An abstract class is the foundation for another object. When a class says "I extend abstract class Y", it is saying "I use some methods or properties already defined in this other class named Y".

    So, consider the following PHP:
    <?php
    class X implements Y { } // this is saying that "X" agrees to speak language "Y" with your code.

    class X extends Y { } // this is saying that "X" is going to complete the partial class "Y".
    ?>

    You would have your class implement a particular interface if you were distributing a class to be used by other people. The interface is an agreement to have a specific set of public methods for your class.

    You would have your class extend an abstract class if you (or someone else) wrote a class that already had some methods written that you want to use in your new class.

    These concepts, while easy to confuse, are specifically different and distinct. For all intents and purposes, if you're the only user of any of your classes, you don't need to implement interfaces.
    mbajoras at gmail dot com
    6 years ago
    Here's an example that helped me with understanding abstract classes. It's just a very simple way of explaining it (in my opinion). Lets say we have the following code:

    <?php
    class Fruit {
        private     $color;
       
        public function     eat() {
                //chew
            }
       
        public function     setColor($c) {
                $this->color = $c;
        }
    }

    class     Apple extends Fruit {
        public function     eat() {
                //chew until core
            }
    }

    class     Orange extends Fruit {
        public function     eat() {
                //peel
            //chew
            }
    }
    ?>

    Now I give you an apple and you eat it.

    <?php
    $apple     = new Apple();
    $apple->eat();
    ?>

    What does it taste like? It tastes like an apple. Now I give you a fruit.

    <?php
    $fruit     = new Fruit();
    $fruit->eat();
    ?>

    What does that taste like??? Well, it doesn't make much sense, so you shouldn't be able to do that. This is accomplished by making the Fruit class abstract as well as the eat method inside of it.

    <?php
    abstract class Fruit {
        private     $color;
       
        abstract public function     eat();
       
        public function     setColor($c) {
                $this->color = $c;
        }
    }
    ?>

    Now just think about a Database class where MySQL and PostgreSQL extend it. Also, a note. An abstract class is just like an interface, but you can define methods in an abstract class whereas in an interface they are all abstract.
    joelhy
    4 years ago
    The documentation says: "It is not allowed to create an instance of a class that has been defined as abstract.". It only means you cannot initialize an object from an abstract class. Invoking static method of abstract class is still feasible. For example:
    <?php
    abstract class Foo
    {
        static function     bar()
        {
            echo     "test\n";
        }
    }

    Foo::bar();
    ?>
    a dot tsiaparas at watergate dot gr
    5 years ago
    Abstraction and interfaces are two very different tools. The are as close as hammers and drills. Abstract classes may have implemented methods, whereas interfaces have no implementation in themselves.

    Abstract classes that declare all their methods as abstract are not interfaces with different names. One can implement multiple interfaces, but not extend multiple classes (or abstract classes).

    The use of abstraction vs interfaces is problem specific and the choice is made during the design of software, not its implementation. In the same project you may as well offer an interface and a base (probably abstract) class as a reference that implements the interface. Why would you do that?

    Let us assume that we want to build a system that calls different services, which in turn have actions. Normally, we could offer a method called execute that accepts the name of the action as a parameter and executes the action.

    We want to make sure that classes can actually define their own ways of executing actions. So we create an interface IService that has the execute method. Well, in most of your cases, you will be copying and pasting the exact same code for execute.

    We can create a reference implemention for a class named Service and implement the execute method. So, no more copying and pasting for your other classes! But what if you want to extend MySLLi?? You can implement the interface (copy-paste probably), and there you are, again with a service. Abstraction can be included in the class for initialisation code, which cannot be predefined for every class that you will write.

    Hope this is not too mind-boggling and helps someone. Cheers,
    Alexios Tsiaparas
    sam at righthandedmonkey dot com
    1 year ago
    Please note order or positioning of the classes in your code can affect the interpreter and can cause a Fatal error: Class 'YourClass' not found if there are multiple levels of abstraction out of order.  For example:
    <?php
    abstract class horse extends animal {
        public function     get_breed() { return "Jersey"; }
    }

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

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

    this outputs:
    Wood

    However, if you put the cart before the abstract horse (literally):

    <?php
    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());
    ?>

    this throws an error:
    Fatal error: Class 'horse' not found

    So, when using multiple levels of abstraction, be careful of the positioning of the classes within the source code - and don't put the cart before the abstract horse.
    pete at surfaceeffect dot com
    6 years ago
    One fairly important difference between php's abstract functions and, say, Java, is that php does not specify the return type in any way - or indeed whether there has to be one.

    <?php public abstract function square($number); ?>

    could be implemented by...

    <?php
    public function square($number) {
       return     $number*$number;
    }
    ?>

    or

    <?php
    public function square($number) {
       print (    $number*$number);
    }
    ?>

    So you need to take care that incompatibilities don't arise due to not returning the right kind of value and this is not enforced in any way.
    bishop
    5 years ago
    Incidentally, abstract classes do not need to be base classes:

    <?php
    class Foo {
        public function     sneeze() { echo 'achoooo'; }
    }

    abstract class     Bar extends Foo {
        public abstract function     hiccup();
    }

    class     Baz extends Bar {
        public function     hiccup() { echo 'hiccup!'; }
    }

    $baz = new Baz();
    $baz->sneeze();
    $baz->hiccup();
    ?>
    balbuf
    3 months ago
    The abstract keyword cannot be used to dictate properties or class constants that a derivative class must set/define. Instead, those required properties or constants can be included in the abstract class with the expectation that they will be overridden in derivative classes, which at least ensures that the desired property/constant is set/defined.
    arma99eDAN at yahoo dot com
    1 year ago
    You can use an abstract class like this too:

    abstract class A{
        public function show(){
            echo 'A';
        }
    }
    class B extends A{
        public function hello(){
            echo 'B';
            parent::show();
        }
    }

    $obj = new B;
    $obj->hello(); // BA
    # See that the abstract class does not have at least one abstract method
    # Even in this case, I'm still able to extend it, or call its non-abstract member
    aloydev2586 at gmail dot com
    1 year ago
    Just in case you are confused about function arguments:

    /*************Example 1********************/

    abstract class my_class {
         abstract public function my_function($number);
    }

    class subclass extends my_class {
         public function my_function($new_number, $string = ' is an integer!!!')
         {
              echo $new_number . $string;
         }
    }
    $var = new subclass();
    $var->my_function(1024); //this will output: 1024 is an integer!!!

    /*************Example 2********************/
    abstract class my_class {
         abstract public function my_function($number);
    }

    class subclass extends my_class {
        //now $string = ' is a float!!!'
         public function my_function($new_number, $string = ' is a float!!!')
         {
              echo $new_number . $string;
         }

    }
    $var = new subclass();
    //added ' is an integer'
    $var->my_function(1024, ' is an integer!!!'); //this will output: 1024 is an integer!!!, rewrote $string.

    /*************Example 3********************/

    abstract class my_class {
         abstract public function my_function($number);
    }

    class subclass extends my_class {
         //now $string isn't initialized
         public function my_function($new_number, $string )
         {
              echo $new_number . $string;
         }

    }

    $var = new subclass();
    $var->my_function(1024, ' is an integer!!!'); /*this will trigger a fatal error of incompatibility between subclass::my_function() and my_class::my_function($number)*/

    /*************Example 4********************/

    abstract class my_class {
         abstract public function my_function($number);
    }

    class subclass extends my_class {   
         public function my_function($new_number, $string )
         {
              echo $new_number . $string;
         }

    }
    $var = new subclass();
    //no second argument, no matter
    $var->my_function(1024);//fatal error  too. Optional arguments have to be initialized in the extending class function.
    joebert
    8 years ago
    I don't agree with jfkallens' last comparison between Abstract Classes & Object Interfaces completely.

    In an Abstract Class, you can define how some methods work, where as in an Object Interface you can not.

    An Object Interface is essentually nothing but a list of function names that a class must define if the class implements that interface.

    An Abstract Class is essentually a prototype which hints towards what extending classes should be doing.
    An Abstract Class can also be thought of as a Base Class that provides some basic functionality, & also defines a built-in Object Interface that all extending classes will implement.

    So, an Object Interface is really a built-in part of an Abstract Class.
    designbyjeeba at gmail dot com
    5 years ago
    Please be aware of the visibility of the parent fields. If the fields are private, then you are not going to see those fields in their childrens. Its basic OOP, but can be problematic sometimes.
    eeescalona
    7 years ago
    here is a real world example of abstract using:

    a (abstract) person class
    a student and an employee final class, which extends person class.

    simple theory is that both student and employee is an extension of the person class.  the difference lies on which table the data is written on, and what other pre processing (ie mandatory field checking, type checking, etc.) needed before writing each of the classes.

    codes:

    <?php

    abstract class person {
       
        abstract protected function     write_info();
       
        public     $LastName;
        public     $FirstName;
        public     $BirthDate;
       
        public function     get_Age($today=NULL){
                //age computation function
            }
    }

    final class     employee extends person{
        public     $EmployeeNumber;
        public     $DateHired;

        public function     write_info(){
            echo     "Writing ". $this->LastName . "'s info to emloyee dbase table";   
                //ADD unique mandatory checking unique to EMPLOYEE ONLY
            //actual sql codes here
            }
    }

    final class     student extends person{
        public     $StudentNumber;
        public     $CourseName;
       
        public function     write_info(){
            echo     "Writing ". $this->LastName . "'s info to student dbase table";
                //ADD unique mandatory checking unique to STUDENT ONLY
            //actual sql codes here
            }
    }

    ///----------
    $personA = new employee;
    $personB = new student;

    $personA->FirstName="Joe";
    $personA->LastName="Sbody";

    $personB->FirstName="Ben";
    $personB->LastName="Dover";

    $personA->write_info();
    ?>

    OUTPUT:Writing Sbody's info to emloyee dbase table
    sneakyimp at hotmail dot com
    8 years ago
    Ok...the docs are a bit vague when it comes to an abstract class extending another abstract class.  An abstract class that extends another abstract class doesn't need to define the abstract methods from the parent class.  In other words, this causes an error:

    <?php
    abstract class class1 {
      abstract public function     someFunc();
    }
    abstract class     class2 extends class1 {
      abstract public function     someFunc();
    }
    ?>

    Error: Fatal error: Can't inherit abstract function class1::someFunc() (previously declared abstract in class2) in /home/sneakyimp/public/chump.php on line 7

    However this does not:

    <?php
    abstract class class1 {
      abstract public function     someFunc();
    }
    abstract class     class2 extends class1 {
    }
    ?>

    An abstract class that extends an abstract class can pass the buck to its child classes when it comes to implementing the abstract methods of its parent abstract class.
    nikola at petkanski dot com
    2 years ago
    Invoking static method of abstract class should be removed.

    What interfaces are?
    - A mean to ensure all implementation have the same methods implemented.

    What is an abstract class?
    - It is a interface that can also include some concrete methods.

    Is it right for the developer to be able to invoke a static method of an interface?
    - I think not.

    The GoF teach us to rely on abstract classes and interfaces to hide differences between subclasses from clients.
    - Interface defines an object’s use (protocol)
    - Implementation defines particular policy

    I don't think one should be able to call some abstract logic that is defined inside an abstract class, without even inheriting the class itself.
    Cheese Doodle
    6 years ago
    There isn't really that much of a great hurdle in understanding these things, there really isn't.

    If you're defining a new class that is abstract, it means that you can make some non-abstract functions that you can use to define the general underlying behavior of that class along side abstract ones.

    In interfaces, you can't do that since functions defined therewithin cannot have a body.

    Abstract functions you use for classes that must define more specific behavior when "extending" your class.

    So for a crude example - define by your non-abstract functions how that particular object (which may be part of a larger class hierarchy) would store and process it's data in SQL, XML, etc.

    Then define abstract functions which allow someone implementing that class to specifically manipulate the data that is to be stored. Then require a format which this data must be returned in, and then in your non-abstract functions call those functions on destruction, in normal runtime, and so on.

    Again, non-abstract functions, or even another class could implement the finer points of ensuring that data is in the correct format, and so on, ad infinitum.

    It isn't too much of a reach to say that if you used a normal class instead of an abstract class, then there isn't much intrinsic requirement between the two classes.

    Assuming that you wanted the functions to use each-others functions and you'd need to use them specifically by name, you'd have to write some code which checked to see -- lamely using function_exists() and other lamery -- if that class has the function you require for interoperability, when you could avoid all possible confusion and headaches by simply using the right tool for the job.

    And reading a decent OOP book.
    oliver at ananit dot de
    4 years ago
    Abstract classes may have an final constructor, and sometime it makes sense to implement a class with a final constructor.

    <?php
    abstract class AbstractModel
    {
            //our models must use the default constuctor
                public final function __construct(){}
        public function     inject($array){
            foreach(    array_keys(get_class_vars(get_called_class()))  as $property){
                    $this->$property = $array[$property];
            }
        }
    }

    class     ProductModel extends AbstractModel
    {
        public     $name;
        public     $price;
        protected     $id;
       
        public function     getId(){return $this->id;}
    }

    class     Factory{
        private     $dataSource;
        public function     __consruct($dataSource){
                $this->dataSource = $dataSource;
        }
       
        public function     get($class, $table, $filter, $orderby, $limit){
                $result = array();
            foreach(    $datasource->fetchAssoc($table, $filter, $orderby, $limit) as $rawData){
                    $obj = new $class();//this can only work if ALL models have a default constructor
                    $obj->inject($rawData);
                    $result[] = $obj;
            }
            return     $result;
        }
    }
    ?>

    Note: This is a very simple example, and I am aware that there are other (better) ways to do this.
    Oliver Anan
    passerbyxp at gmail dot com
    3 years ago
    It's not quite clear in this document, so in case someone wonder, abstract method can be static:

    <?php
    abstract class FOO
    {
        abstract static function     dump();
    }
    class     BAR extends FOO
    {
        static function     dump(){var_dump("BAR");}
    }
    BAR::dump(); // string(3) "BAR"

    class FAULT extends FOO
    {
        function     pr(){var_export("WTF");}
    }     // FATAL error
    ?>
    nathan dot vorbei dot tech at gmail dot com
    6 years ago
    "additionally, these methods must be defined with the same (or a less restricted) visibility."

    The words were not restricted in abstract class but also normal classes,
    the method in child Class which overwrites the parent Class can also change the the visibility of the method to same or less restricted.
    for example:
    <?php
    class ClassOne {
        protected static     $staticone = 'nathan';
        protected function     changestaticone() {
            return     self::$staticone = 'john';
        }
    }

    class     ClassTwo extends ClassOne {
        public function     changestaticone() {
            return     self::$staticone = 'Alexey';
        }
    }
    $classtwo = new ClassTwo();
    echo     $classtwo->changestaticone();
    rmoisto at gmail dot com
    7 months ago
    The self keyword in an abstract class will refer to the abstract class itself, not the extending class no matter what.

    For instance the following code looks really pretty, yet results in a Fatal error (Cannot instantiate abstract class Basic).

    <?php
    abstract class Basic {
        public static function     doWork() {
            return (new     self())->work();
        }

        abstract protected function     work();
    }

    class     Keeks extends Basic {
        protected function     work() {
            return     'Keeks';
        }
    }

    echo     Keeks::doWork();
    ?>
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