How to declare abstract method in non-abstract class in PHP?

Question

class absclass {
    abstract public function fuc();
}

reports:

PHP Fatal error: Class absclass contains 1 abstract metho

Answer 1

An abstract class cannot be directly instantiated, but it can contain both abstract and non-abstract methods.

If you extend an abstract class, you have to either implement all its abstract functions, or make the subclass abstract.

You cannot override a regular method and make it abstract, but you must (eventually) override all abstract methods and make them non-abstract.

name = $name;
        $this->gender = $gender;
    }

    public function getName() {return $this->name;}
    public function setName($name) {$this->name = $name;}
    public function getGender() {return $this->gender;}
    public function setGender($gender) {$this->gender = $gender;}

    abstract public function bark();

}

// non-abstract class inheritting from an abstract class - this one has to implement all inherited abstract methods.
class Daschund extends Dog {
    public function bark() {
        print "bowowwaoar" . PHP_EOL;
    }
}

// this class causes a compilation error, because it fails to implement bark().
class BadDog extends Dog {
    // boom!  where's bark() ?
}

// this one succeeds in compiling, 
// it's passing the buck of implementing it's inheritted abstract methods on to sub classes.
abstract class PassTheBuckDog extends Dog {
    // no boom. only non-abstract subclasses have to bark(). 
}

$dog = new Daschund('Fred', 'male');
$dog->setGender('female');

print "name: " . $dog->getName() . PHP_EOL;
print "gender: ". $dog->getGender() . PHP_EOL;

$dog->bark();

?>

That program bombs with:

PHP Fatal error: Class BadDog contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Dog::bark)

If you comment out the BadDog class, then the output is:

name: Fred
gender: female
bowowwaoar

If you try to instantiate a Dog or a PassTheBuckDog directly, like this:

$wrong = new Dog('somma','it');
$bad = new PassTheBuckDog('phamous','monster');

..it bombs with:

PHP Fatal error: Cannot instantiate abstract class Dog

or (if you comment out the $wrong line)

PHP Fatal error: Cannot instantiate abstract class PassTheBuckDog

You can, however, call a static function of an abstract class:

abstract class Dog {
    ..
    public static function getBarker($classname, $name, $gender) {
        return new $classname($name, $gender);
    }
    ..
}

..

$other_dog = Dog::getBarker('Daschund', 'Wilma', 'female');
$other_dog->bark();

That works just fine.