Trait vs inheritance

PHP course · Intermediate · Lesson 63/115

Learn the difference between trait and inheritance and when to use each.

Trait vs inheritance: vertical or horizontal?

In PHP inheritance is vertical: a class derives from a single parent and forms an 'is-a' relationship. When you write Dog extends Animal, you say 'a Dog is an Animal'.

A trait is horizontal: instead of 'is-a', it says 'has this capability'. Unrelated classes can share the same trait. And a class can use many traits, while inheriting from only one class.

When a trait, when inheritance?

When to use which? If there's a real type hierarchy, choose inheritance: Cat extends Animal, Car extends Vehicle. The subclass is a specialized parent.

If a behavior is shared across classes without kinship, choose a trait: both Robot and Phone can use a Battery trait; there's no need to tie them to a common parent.

Exercise Horizontal reuse in two unrelated classes

Define a Timestamp trait: its public function created(): string method does return "created";. Then Article and Comment classes use the trait with use Timestamp; (neither extends a parent; they are unrelated). Create instances with $a = new Article() and $c = new Comment(); call echo $a->created() . "\n" and echo $c->created() . "\n".

Show example solution

string { ... } }. Then add class Article { use Timestamp; } and class Comment { use Timestamp; }. Create both instances and call created().
Example solution:
<?php
// Use the Timestamp trait in two unrelated classes
trait Timestamp {
    public function created(): string {
        return "created";
    }
}

class Article {
    use Timestamp;
}

class Comment {
    use Timestamp;
}

$a = new Article();
$c = new Comment();
echo $a->created() . "\n";
echo $c->created() . "\n";

Test yourself

Which statement about traits and inheritance is true?
- A class can inherit multiple classes but use only one trait
- A class can use multiple traits but inherit only one class
- Traits and inheritance are exactly the same
- Using a trait makes extends unnecessary
Show answer

A class can use multiple traits but inherit only one class — Correct. PHP has single inheritance, but a class can use any number of traits; this gives horizontal reuse.

Include a trait in an unrelated class

trait Battery {
    public function charge(): string {
        return "charging";
    }
}

class Robot {
    ____ Battery;
}

$r = new Robot();
echo $r->charge() . "\n";

Show answer

trait Battery {
    public function charge(): string {
        return "charging";
    }
}

class Robot {
    use Battery;
}

$r = new Robot();
echo $r->charge() . "\n";

Match the concept to its category

Dog extends Animal

inheritance (is-a)
class + use Battery

horizontal reuse
single parent limit

PHP inheritance rule
multiple trait combine

trait advantage

Trait vs inheritance