Chapter 10 · Going Deeper

Traits

PHP allows a class to extend only one parent. But what if two completely unrelated classes need the same handy methods? Copy-paste? Never. PHP's answer is the trait — a reusable package of methods you can drop into any class.

The problem traits solve

Imagine both Student and Invoice need timestamp tracking — "when was this created? when was it updated?". They are totally unrelated: a student is not an invoice, an invoice is not a student, so inheritance makes no sense. Yet the code they need is identical.

Writing and using a trait

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

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

    public function touchUpdated(): void {
        $this->updatedAt = date("Y-m-d H:i:s");
    }
}

class Student {
    use HasTimestamps;    // install the ability-pack

    public function __construct(public string $name) {
        $this->touchCreated();
    }
}

class Invoice {
    use HasTimestamps;    // same pack, totally different class

    public function __construct(public float $amount) {
        $this->touchCreated();
    }
}

$s = new Student("Chanda");
echo "Student created at: $s->createdAt <br>";

$inv = new Invoice(5000);
echo "Invoice created at: $inv->createdAt";

Note the details:

• Declared with trait Name { ... } — looks exactly like a class body.
• Installed with use TraitName; as the first line inside a class.
• Once installed, its methods and properties behave as if you had typed them directly into the class — $this works normally.
• A trait can never be instantiated: new HasTimestamps() is an error. It's an ability-pack, not a thing.
• (The ?string type means "a string or null" — null until the timestamp is first set.)

Multiple traits in one class

trait CanLog {
    public function log(string $msg): void {
        echo "[" . date("H:i:s") . "] " . get_class($this) . ": $msg<br>";
    }
}

trait HasSlug {
    public function slug(string $text): string {
        return strtolower(str_replace(" ", "-", trim($text)));
    }
}

class BlogPost {
    use CanLog, HasSlug;      // install two packs at once

    public function __construct(public string $title) {
        $this->log("Post created: " . $this->slug($title));
    }
}

new BlogPost("Learning PHP OOP Is Fun");

(get_class($this) is a handy built-in that returns the name of the current object's class — great for log messages.)

Name clashes — when two traits collide

If two installed traits both define a method called hello(), PHP stops and asks you to choose. You resolve it with insteadof (pick one) and optionally as (keep the other under a new name):

trait EnglishGreeting { public function hello(): string { return "Hello!"; } }
trait BembaGreeting   { public function hello(): string { return "Muli shani!"; } }

class Receptionist {
    use EnglishGreeting, BembaGreeting {
        BembaGreeting::hello insteadof EnglishGreeting;  // Bemba wins the name "hello"
        EnglishGreeting::hello as helloEnglish;          // English survives under a new name
    }
}

$r = new Receptionist();
echo $r->hello() . " / " . $r->helloEnglish();

You won't need this often — but when the error "trait method collision" appears, you'll know exactly what it means and how to fix it.

Trait vs inheritance vs interface — the cheat sheet

<?php
trait Describable {
    public function describe(): string {
        return get_class($this) . " object ready!";
    }
}

class Car   { use Describable; }
class Pizza { use Describable; }

echo (new Car())->describe() . "<br>";
echo (new Pizza())->describe();