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 ezcReflectionReturnInfo {
function
getReturnType() { /*1*/ }
function
getReturnDescription() { /*2*/ }
}

class
ezcReflectionMethod extends ReflectionMethod {
use
ezcReflectionReturnInfo;
/* ... */
}

class
ezcReflectionFunction extends ReflectionFunction {
use
ezcReflectionReturnInfo;
/* ... */
}
?>

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();
?>

The above example will output:

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();
?>

The above example will output:

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();
?>

The above example will output:

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();
?>

The above example will output:

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.

Caution

A concrete class fulfills this requirement by defining a concrete method with the same name; its signature may 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.

Note:

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(); // echo 1
$p = new C2(); $p->inc(); // echo 1
?>

Example #10 Static Methods

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

class
Example {
use
StaticExample;
}

Example::doSomething();
?>

Example #11 Static Properties

<?php
trait StaticExample {
public static
$static = 'foo';
}

class
Example {
use
StaticExample;
}

echo
Example::$static;
?>

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 public bool $different3; // Fatal error
}
?>

Constants

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; // 1
?>

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
}
?>
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