PHP OOP: Abstract Classes, Interfaces, and Namespaces

2 min read

After mastering Classes, Objects, and Inheritance, these three concepts—Abstract Classes, Interfaces, and Namespaces—provide the necessary framework for organizing, defining, and enforcing structure across large, complex applications.

1. Abstract Classes (Incomplete Blueprints)

An Abstract Class is a class that cannot be instantiated (you can’t create an object directly from it). It is designed only to be a parent class that provides a base structure and required methods for its children.

Key Idea: It serves as a half-finished template that forces child classes to fill in the missing methods.
Keyword: Use the abstract keyword to define both the class and any methods within it that must be completed by the child.

Example: Abstract Vehicle Class

<?php
// Abstract Class (cannot be instantiated on its own)
abstract class Vehicle {
    protected $speed;

    // This method MUST be implemented by any child class (Car, Truck, etc.)
    abstract public function startEngine();

    public function increaseSpeed($amount) {
        $this->speed += $amount;
        return "Speed increased to " . $this->speed . " mph.";
    }
}

// Child Class must implement the abstract method
class Car extends Vehicle {
    public function startEngine() {
        return "Car ignition successful. Ready to drive.";
    }
}

$myCar = new Car();
echo $myCar->startEngine() . "<br>";
echo $myCar->increaseSpeed(60); 
// Output:
// Car ignition successful. Ready to drive.
// Speed increased to 60 mph.
// $myVehicle = new Vehicle(); // ERROR: Cannot instantiate abstract class!
?>

2. Interfaces (Contract and Guarantee)

An Interface is not a class; it is a contract or a blueprint of methods that a class must implement. It contains no properties and no actual code—only method declarations.

Key Idea: It guarantees that any class using the interface will have specific methods available, regardless of what the class does internally.
Keyword: Use the interface keyword to define it and the implements keyword in the class that uses it.

Example: Enforcing a Loggable Contract

<?php
// 1. Define the Interface (The Contract)
interface Loggable {
    // The class using this interface MUST implement these two methods
    public function getLogMessage();
    public function saveToDatabase();
}

// 2. Implement the Interface (Fulfilling the Contract)
class ErrorReport implements Loggable {
    public $errorMessage = "Server down at 13:00.";

    public function getLogMessage() {
        return "ERROR: " . $this->errorMessage;
    }

    public function saveToDatabase() {
        // Code to securely insert $this->errorMessage into a database table
        return "Report saved successfully.";
    }
}

$report = new ErrorReport();
echo $report->getLogMessage() . "<br>"; // Guaranteed to exist by the interface
echo $report->saveToDatabase();
?>

Note: A class can implement multiple interfaces, but it can only inherit from one abstract or non-abstract class.

3. Namespaces (Avoiding Name Collisions)

As applications grow, different libraries or parts of your code might use the same class name (e.g., a User class in a security library and another User class in a reporting library). Namespaces solve this problem by providing a container to logically group related classes.

Key Idea: Namespaces are like the directory structure on your computer—they allow you to have multiple files with the same name as long as they are in different folders.
Keywords: Use the namespace keyword to define the container and the use keyword to import classes from other namespaces.

Example:

<?php
// FILE: /App/Database/Connection.php
namespace App\Database;

class Connection {
    public function connect() { return "Connected via App\\Database\\Connection."; }
}

// FILE: /App/Service/Connection.php (Different class, same name)
namespace App\Service;

class Connection {
    public function connect() { return "Connected via App\\Service\\Connection."; }
}

// FILE: index.php (Usage)
use App\Database\Connection as DBConnection; // Alias the first class
use App\Service\Connection as ServiceConnection; // Alias the second class

$db = new DBConnection();
$service = new ServiceConnection();

echo $db->connect() . "<br>";
echo $service->connect();
// Output:
// Connected via App\Database\Connection.
// Connected via App\Service\Connection.
?>

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