Abstract classes and abstract methods in PHP

 In PHP, abstract classes and abstract methods allow you to define a blueprint for other classes. They’re especially useful in cases where you want to enforce that a class should have specific methods, but the actual implementation of those methods can vary between classes.

### Abstract Classes
An abstract class in PHP is a class that cannot be instantiated on its own. Instead, it must be extended by other classes. Abstract classes can include both abstract methods (methods without a body) and concrete methods (methods with a body).

### Abstract Methods
An abstract method is a method that is declared but doesn’t have an implementation. Any class that extends an abstract class with an abstract method must implement that method.

### Example
Here’s a simple example to illustrate abstract classes and methods in PHP:

```php
<?php
// Define an abstract class
abstract class Animal {
    // Abstract method (must be implemented by subclasses)
    abstract public function makeSound();

    // Concrete method (can be used as is by subclasses)
    public function move() {
        echo "The animal moves.\n";
    }
}

// Define a subclass that extends the abstract class
class Dog extends Animal {
    // Implement the abstract method
    public function makeSound() {
        echo "Bark!\n";
    }
}

// Define another subclass that extends the abstract class
class Cat extends Animal {
    // Implement the abstract method
    public function makeSound() {
        echo "Meow!\n";
    }
}

// Create instances of the subclasses
$dog = new Dog();
$cat = new Cat();

// Call methods
$dog->makeSound();  // Output: Bark!
$dog->move();       // Output: The animal moves.

$cat->makeSound();  // Output: Meow!
$cat->move();       // Output: The animal moves.
?>
```

### Explanation
1. The `Animal` class is an abstract class. It includes an abstract method `makeSound()` that doesn’t have a body, as well as a concrete method `move()` that does.
2. The `Dog` and `Cat` classes extend `Animal`. Both of these classes implement the `makeSound()` method, providing their own output.
3. When you create instances of `Dog` and `Cat` and call `makeSound()` and `move()`, they behave according to their specific implementations.

### Key Points
- **Abstract classes** can contain both abstract and concrete methods.
- **Abstract methods** must be implemented in any non-abstract subclass.
- You cannot create an instance of an abstract class directly; it must be extended.