Traits

PHP implements a way to reuse code called Traits.

Traits are a mechanism for code reuse in single inheritance languages such as PHP. A Trait is intended to reduce some limitations of single inheritance by enabling a developer to reuse sets of methods freely in several independent classes living in different class hierarchies. The semantics of the combination of Traits and classes is defined in a way which reduces complexity, and avoids the typical problems associated with multiple inheritance and Mixins.

A Trait is similar to a class, but only intended to group functionality in a fine-grained and consistent way. It is not possible to instantiate a Trait on its own. It is an addition to traditional inheritance and enables horizontal composition of behavior; that is, the application of class members without requiring inheritance.

Example #1 Trait example

<?php

trait TraitA {
    public function sayHello() {
        echo 'Hello';
    }
}

trait TraitB {
    public function sayWorld() {
        echo 'World';
    }
}

class MyHelloWorld
{
    use TraitA, TraitB; // A class can use multiple traits

    public function sayHelloWorld() {
        $this->sayHello();
        echo ' ';
        $this->sayWorld();
        echo "!\n";
    }
}

$myHelloWorld = new MyHelloWorld();
$myHelloWorld->sayHelloWorld();

?>

Il precedente esempio visualizzerà:

Hello World!

Precedence

An inherited member from a base class is overridden by a member inserted by a Trait. The precedence order is that members from the current class override Trait methods, which in turn override inherited methods.

Example #2 Precedence Order Example

An inherited method from a base class is overridden by the method inserted into MyHelloWorld from the SayWorld Trait. The behavior is the same for methods defined in the MyHelloWorld class. The precedence order is that methods from the current class override Trait methods, which in turn override methods from the base class.

<?php
class Base {
    public function sayHello() {
        echo 'Hello ';
    }
}

trait SayWorld {
    public function sayHello() {
        parent::sayHello();
        echo 'World!';
    }
}

class MyHelloWorld extends Base {
    use SayWorld;
}

$o = new MyHelloWorld();
$o->sayHello();
?>

Il precedente esempio visualizzerà:

Hello World!

Example #3 Alternate Precedence Order Example

<?php
trait HelloWorld {
    public function sayHello() {
        echo 'Hello World!';
    }
}

class TheWorldIsNotEnough {
    use HelloWorld;
    public function sayHello() {
        echo 'Hello Universe!';
    }
}

$o = new TheWorldIsNotEnough();
$o->sayHello();
?>

Il precedente esempio visualizzerà:

Hello Universe!

Multiple Traits

Multiple Traits can be inserted into a class by listing them in the use statement, separated by commas.

Example #4 Multiple Traits Usage

<?php
trait Hello {
    public function sayHello() {
        echo 'Hello ';
    }
}

trait World {
    public function sayWorld() {
        echo 'World';
    }
}

class MyHelloWorld {
    use Hello, World;
    public function sayExclamationMark() {
        echo '!';
    }
}

$o = new MyHelloWorld();
$o->sayHello();
$o->sayWorld();
$o->sayExclamationMark();
?>

Il precedente esempio visualizzerà:

Hello World!

Conflict Resolution

If two Traits insert a method with the same name, a fatal error is produced, if the conflict is not explicitly resolved.

To resolve naming conflicts between Traits used in the same class, the insteadof operator needs to be used to choose exactly one of the conflicting methods.

Since this only allows one to exclude methods, the as operator can be used to add an alias to one of the methods. Note the as operator does not rename the method and it does not affect any other method either.

Example #5 Conflict Resolution

In this example, Talker uses the traits A and B. Since A and B have conflicting methods, it defines to use the variant of smallTalk from trait B, and the variant of bigTalk from trait A.

The Aliased_Talker makes use of the as operator to be able to use B's bigTalk implementation under an additional alias talk.

<?php
trait A {
    public function smallTalk() {
        echo 'a';
    }
    public function bigTalk() {
        echo 'A';
    }
}

trait B {
    public function smallTalk() {
        echo 'b';
    }
    public function bigTalk() {
        echo 'B';
    }
}

class Talker {
    use A, B {
        B::smallTalk insteadof A;
        A::bigTalk insteadof B;
    }
}

class Aliased_Talker {
    use A, B {
        B::smallTalk insteadof A;
        A::bigTalk insteadof B;
        B::bigTalk as talk;
    }
}
?>

Changing Method Visibility

Using the as syntax, one can also adjust the visibility of the method in the exhibiting class.

Example #6 Changing Method Visibility

<?php
trait HelloWorld {
    public function sayHello() {
        echo 'Hello World!';
    }
}

// Change visibility of sayHello
class MyClass1 {
    use HelloWorld { sayHello as protected; }
}

// Alias method with changed visibility
// sayHello visibility not changed
class MyClass2 {
    use HelloWorld { sayHello as private myPrivateHello; }
}
?>

Traits Composed from Traits

Just as classes can make use of traits, so can other traits. By using one or more traits in a trait definition, it can be composed partially or entirely of the members defined in those other traits.

Example #7 Traits Composed from Traits

<?php
trait Hello {
    public function sayHello() {
        echo 'Hello ';
    }
}

trait World {
    public function sayWorld() {
        echo 'World!';
    }
}

trait HelloWorld {
    use Hello, World;
}

class MyHelloWorld {
    use HelloWorld;
}

$o = new MyHelloWorld();
$o->sayHello();
$o->sayWorld();
?>

Il precedente esempio visualizzerà:

Hello World!

Abstract Trait Members

Traits support the use of abstract methods in order to impose requirements upon the exhibiting class. Public, protected, and private methods are supported. Prior to PHP 8.0.0, only public and protected abstract methods were supported.

Attenzione

As of PHP 8.0.0, the signature of a concrete method must follow the signature compatibility rules. Previously, its signature might be different.

Example #8 Express Requirements by Abstract Methods

<?php
trait Hello {
    public function sayHelloWorld() {
        echo 'Hello'.$this->getWorld();
    }
    abstract public function getWorld();
}

class MyHelloWorld {
    private $world;
    use Hello;
    public function getWorld() {
        return $this->world;
    }
    public function setWorld($val) {
        $this->world = $val;
    }
}
?>

Static Trait Members

Traits can define static variables, static methods and static properties.

Nota:
As of PHP 8.1.0, calling a static method, or accessing a static property directly on a trait is deprecated. Static methods and properties should only be accessed on a class using the trait.

Example #9 Static Variables

<?php

trait Counter
{
    public function inc()
    {
        static $c = 0;
        $c = $c + 1;
        echo "$c\n";
    }
}

class C1
{
    use Counter;
}

class C2
{
    use Counter;
}

$o = new C1();
$o->inc();
$p = new C2();
$p->inc();

?>

Il precedente esempio visualizzerà:

1
1

Example #10 Static Methods

<?php

trait StaticExample
{
    public static function doSomething()
    {
        return 'Doing something';
    }
}

class Example
{
    use StaticExample;
}

echo Example::doSomething();

?>

Il precedente esempio visualizzerà:

Doing something

Example #11 Static Properties

Attenzione

Prior to PHP 8.3.0, static properties defined in a trait were shared across all classes in the same inheritance hierarchy which used that trait. As of PHP 8.3.0, if a child class uses a trait with a static property, it will be considered distinct from the one defined in the parent class.

<?php

trait T
{
    public static $counter = 1;
}

class A
{
    use T;

    public static function incrementCounter()
    {
        static::$counter++;
    }
}

class B extends A
{
    use T;
}

A::incrementCounter();

echo A::$counter, "\n";
echo B::$counter, "\n";

?>

Output of the above example in PHP 8.3:

2
1

Properties

Traits can also define properties.

Example #12 Defining Properties

<?php

trait PropertiesTrait
{
    public $x = 1;
}

class PropertiesExample
{
    use PropertiesTrait;
}

$example = new PropertiesExample();
$example->x;

?>

If a trait defines a property then a class can not define a property with the same name unless it is compatible (same visibility and type, readonly modifier, and initial value), otherwise a fatal error is issued.

Example #13 Conflict Resolution

<?php
trait PropertiesTrait {
    public $same = true;
    public $different1 = false;
    public bool $different2;
    public bool $different3;
}

class PropertiesExample {
    use PropertiesTrait;
    public $same = true;
    public $different1 = true; // Fatal error
    public string $different2; // Fatal error
    readonly protected bool $different3; // Fatal error
}
?>

Costanti

Traits can, as of PHP 8.2.0, also define constants.

Example #14 Defining Constants

<?php
trait ConstantsTrait {
    public const FLAG_MUTABLE = 1;
    final public const FLAG_IMMUTABLE = 5;
}

class ConstantsExample {
    use ConstantsTrait;
}

$example = new ConstantsExample;
echo $example::FLAG_MUTABLE;
?>

Il precedente esempio visualizzerà:

If a trait defines a constant then a class can not define a constant with the same name unless it is compatible (same visibility, initial value, and finality), otherwise a fatal error is issued.

Example #15 Conflict Resolution

<?php
trait ConstantsTrait {
    public const FLAG_MUTABLE = 1;
    final public const FLAG_IMMUTABLE = 5;
}

class ConstantsExample {
    use ConstantsTrait;
    public const FLAG_IMMUTABLE = 5; // Fatal error
}
?>

Final methods

As of PHP 8.3.0, the final modifier can be applied using the as operator to methods imported from traits. This can be used to prevent child classes from overriding the method. However, the class that uses the trait can still override the method.

Example #16 Defining a method coming from a trait as final

<?php

trait CommonTrait
{
    public function method()
    {
        echo 'Hello';
    }
}

class FinalExampleA
{
    use CommonTrait {
        CommonTrait::method as final; // The 'final' prevents child classes from overriding the method
    }
}

class FinalExampleB extends FinalExampleA
{
    public function method() {}
}

?>

Il precedente esempio visualizzerà qualcosa simile a:

Fatal error: Cannot override final method FinalExampleA::method() in ...