PHP Interfaces & Traits

Interfaces

An interface is a contract: a list of methods a class promises to provide. It contains no implementation.

<?php
interface Payable
{
    public function amountDue(): float;
}

interface Refundable
{
    public function refund(): bool;
}

class Order implements Payable, Refundable
{
    public function __construct(private float $total) {}

    public function amountDue(): float { return $this->total; }
    public function refund(): bool     { return true; }
}

A class can implement many interfaces but extend only one class. That is the main reason interfaces exist.

Code can then depend on the contract rather than a concrete class:

<?php
function charge(Payable $item): void
{
    echo "Charging NPR " . $item->amountDue();
}

Anything implementing Payable works here — orders, subscriptions, invoices — without charge() knowing or caring which.

Traits

A trait is a bundle of methods that can be reused by unrelated classes — PHP's answer to "I need this behaviour in two classes that shouldn't share a parent."

<?php
trait Timestampable
{
    public ?string $createdAt = null;

    public function touch(): void
    {
        $this->createdAt = date('Y-m-d H:i:s');
    }
}

class Article { use Timestampable; }
class Comment { use Timestampable; }

$a = new Article();
$a->touch();
echo $a->createdAt;

Multiple traits are fine:

<?php
class Post
{
    use Timestampable, SoftDeletes;
}

If two traits define the same method, PHP raises an error unless you resolve it explicitly with insteadof.

Interface vs trait vs abstract class

Reach for interfaces by default; use traits for genuinely repeated implementation, and be wary of traits that carry a lot of state.