Summary - Class constants

PHP course · Intermediate · Lesson 41/115

Build an order class with status constants using const, :: and ::class.

Summary - Class constants

In this group you learned class constants:

const defines a class constant; no $ sign, immutable.
The :: operator reaches constants from outside with ClassName::CONSTANT and from inside with self::CONSTANT.
::class returns the fully qualified class name as a string.
Constants are immutable; static properties can change.
Great for configuration, status codes and enum-like structures.

Now you'll combine these in an Order class with status constants.

Exercise Order class with status constants

Define an Order class with const string PENDING = 'pending'; and const string SHIPPED = 'shipped';. Add a private string $status property and a public function __construct(string $status = self::PENDING) that assigns $this->status = $status. Add public function label(): string with if ($this->status === self::SHIPPED) { return 'Shipped'; } and a final return 'Pending'. Then create $order = new Order(Order::SHIPPED) and print echo $order->label() . "\n". (Expected output: Shipped)

Show example solution

:PENDING) { $this->status = $status; } public function label(): string { if ($this->status === self::SHIPPED) { return 'Shipped'; } return 'Pending'; } } then $order = new Order(Order::SHIPPED); echo $order->label() . "\n";
Example solution:
<?php
// Define an Order class
class Order {
    const string PENDING = 'pending';
    const string SHIPPED = 'shipped';
    private string $status;
    public function __construct(string $status = self::PENDING) {
        $this->status = $status;
    }
    public function label(): string {
        if ($this->status === self::SHIPPED) {
            return 'Shipped';
        }
        return 'Pending';
    }
}
$order = new Order(Order::SHIPPED);
echo $order->label() . "
";

Tips for class constants

In summary: use const for immutable values and static for mutable ones. Access constants with ::; self:: inside, ClassName:: outside.

Use constants for configuration, status codes and enum-like structures. This makes your code more readable and safe; it avoids the magic number and magic string problems.

In the next group you'll learn to define contracts between classes with interfaces.

Test yourself

Match the class constant concepts

const

Defines a class constant
::

Operator to access constants
::class

Gives the fully qualified name
self::

In-class access
For order status in an e-commerce app, what is the best choice?
- Statuses defined as class constants (const)
- A separate variable for each order
- Writing a new string in the database each time
- Storing it as a public property
Show answer

Statuses defined as class constants (const) — Correct. Order statuses are immutable values; a class constant is ideal.

Summary - Class constants