Using Abstract Base Classes

Abstract classes offer shared behavior and enforce subclass method contracts.

The examples focus on the operation itself, including the important distinction: Interfaces declare contracts without forcing one shared implementation.

What It Does

Use the examples below to observe the behavior on a small dataset before extending it to application code.

Before You Start

Read the input requirements before applying this pattern to production data.

Example 1: Basic Use

The base class supplies shared logic and requires subclasses to implement the missing operation.

<?php
abstract class Notification {
    abstract public function channel(): string;
    public function label(): string { return "Send by " . $this->channel(); }
}
class EmailNotification extends Notification { public function channel(): string { return "email"; } }
echo (new EmailNotification())->label();

Output from a successful local PHP 8.4 run:

Send by email

Example 2: Checking Behavior

The base class supplies shared logic and requires subclasses to implement the missing operation. This distinguishes inherited behavior from the abstract declaration itself.

<?php
abstract class Notification {
    abstract public function channel(): string;
    public function label(): string { return "Send by " . $this->channel(); }
}
class EmailNotification extends Notification { public function channel(): string { return "email"; } }
echo (new EmailNotification())->channel();

Output from a successful local PHP 8.4 run:

email

Example 3: Practical Example

The base class supplies shared logic and requires subclasses to implement the missing operation. This distinguishes inherited behavior from the abstract declaration itself.

<?php
abstract class Notification {
    abstract public function channel(): string;
    public function label(): string { return "Send by " . $this->channel(); }
}
class EmailNotification extends Notification { public function channel(): string { return "email"; } }
echo (new ReflectionClass(Notification::class))->isAbstract() ? "abstract" : "concrete";

Output from a successful local PHP 8.4 run:

abstract

Important Behavior and Edge Cases

An abstract base class cannot be instantiated, and overusing inheritance couples otherwise independent behavior.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Abstract classes offer shared behavior and enforce subclass method contracts.
• An abstract base class cannot be instantiated, and overusing inheritance couples otherwise independent behavior.
• Interfaces declare contracts without forcing one shared implementation.

Use Using Abstract Base Classes when its documented behavior matches the problem. Test the relevant edge cases in your own runtime before deploying it.