PHP Abstract Classes

Abstract Classes & Methods (abstract)

An abstract class is a parent class marked with the abstract keyword that cannot be instantiated directly. It serves as a structural template that forces derived child subclasses to implement declared abstract methods.

Abstract Class Architecture

flowchart TD
    A["abstract class PaymentProcessor"] -->|Declares abstract method| B["abstract public function processPayment()"]
    A --> C["Concrete Subclass 1 (StripeProcessor)"]
    A --> D["Concrete Subclass 2 (PayPalProcessor)"]
    C --> C1["Implements processPayment()"]
    D --> D1["Implements processPayment()"]

Abstract Class Rules

1. Abstract classes cannot be instantiated with new.
2. Abstract methods contain signature declarations only (no code body {}).
3. Subclasses extending an abstract class must implement all abstract methods with matching signature arguments and types.

Practical Code Example

<?php
declare(strict_types=1);

// Abstract Base Class Template
abstract class DatabaseDriver
{
    // Concrete property & method available to all subclasses
    public function __construct(protected string $connectionString) {}

    public function logConnection(): void
    {
        echo "Connecting to driver: {$this->connectionString}
";
    }

    // Abstract method signature (Must be implemented by child classes)
    abstract public function connect(): bool;
    abstract public function query(string $sql): array;
}

class MySQLDriver extends DatabaseDriver
{
    public function connect(): bool
    {
        $this->logConnection();
        return true; // Mock connection
    }

    public function query(string $sql): array
    {
        return [["id" => 1, "result" => "MySQL Query Executed"]];
    }
}

$db = new MySQLDriver("mysql:host=localhost;dbname=test");
$db->connect();
print_r($db->query("SELECT * FROM users"));

Best Practices

• Use Abstract Classes for Shared Concrete Base Logic: Ideal when child subclasses share common properties and helper methods alongside required contract methods.
• Match Signature Visibility & Type Hints: Ensure child implementations match or broaden visibility and strict type signatures.
• Do Not Instantiating Abstract Classes: Attempting new AbstractClass() causes a fatal runtime error.

Self-Check Challenge

Can an abstract class be instantiated directly using the new keyword? (No!)