phpapprentice/code/abstract.php

<?php

// Abstract classes are similar to interfaces in that they define methods that a sub-class must implement.
// However, an abstract class can also have normal methods. To create an abstract class, use the "abstract"
// keyword followed by class and the name of the class.
abstract class CellPhone
{
    abstract public function unlock();

    public function turnOn()
    {
        echo "Holding power button...\n";
    }
}

// To use an abstract class, you create another class that extends it and create any methods that were marked as abstract.
// A class can only extend one abstract class and the child class has to implement all abstract methods.
class iPhone extends CellPhone
{
    public function unlock()
    {
        echo "Touching fingerprint reader...\n";
    }
}

// In this example, we use an abstract class to create the behavior for turning on a cell phone and then
// force the child classes to implement how to unlock the phone. We have clearly defined what a cell phone
// performs and we have limited code duplication.
class Android extends CellPhone
{
    public function unlock()
    {
        echo "Typing passcode...\n";
    }
}

// Our iPhone and Android classes can now both use the turnOn method and the unlock method.
$iPhone = new iPhone();
$iPhone->turnOn();
$iPhone->unlock();

$android = new Android();
$android->turnOn();
$android->unlock();

// Lastly, you cannot create an instance of an abstract class. PHP would not know how to use the abstract methods
// so when you try to create an abstract instance you will get an error.
$cellPhone = new CellPhone();