PHP use abstract class or interface?

Question

In this piece of code is it better to use abstract classes in place of the interfaces or is it good as it currently is? If so, why?

/** contract for all flyable vehicles **/
interface iFlyable {
    public function fly();
}

/* concrete implementations of iFlyable interface */
class JumboJet implements iFlyable {
    public function fly() {
        return "Flying 747!";
    }
}

class FighterJet implements iFlyable {
    public function fly() {
        return "Flying an F22!";
    }
}

class PrivateJet implements iFlyable {
    public function fly() {
        return "Flying a Lear Jet!";
    }
}

/** contract for conrete Factory **/
/**
* "Define an interface for creating an object, but let the classes that implement the interface
* decide which class to instantiate. The Factory method lets a class defer instantiation to
* subclasses."
**/
interface iFlyableFactory {
    public static function create( $flyableVehicle );
}

/** concrete factory **/
class JetFactory implements iFlyableFactory {
    /* list of available products that this specific factory makes */
    private  static $products = array( 'JumboJet', 'FighterJet', 'PrivateJet' );

    public  static function create( $flyableVehicle ) {
        if( in_array( $flyableVehicle, JetFactory::$products ) ) {
            return new $flyableVehicle;
        } else {
            throw new Exception( 'Jet not found' );
        }
    }
}

$militaryJet = JetFactory::create( 'FighterJet' );
$privateJet = JetFactory::create( 'PrivateJet' );
$commercialJet = JetFactory::create( 'JumboJet' );

Answer 1

the interface is more flexible. this way not everything that flies is forced to inherit from the same base class (php doesnt support multiple inheritance)

so

class bird extends animal implements flyable
class plane extends machine implements flyable
class cloud implements flyable

sometimes flexibility is not desired.

abstract classes can also provide function definitions which will reduce code duplication if multiple flying classes required the same fly() method

hope that helps you understand your choice