Old-style constructors

Prior to PHP 8.0.0, classes in the global namespace will interpret a method named the same as the class as an old-style constructor. That syntax is deprecated, and will result in an E_DEPRECATED error but still call that function as a constructor. If both __construct() and a same-name method are defined, __construct() will be called.

In namespaced classes, or any class as of PHP 8.0.0, a method named the same as the class never has any special meaning.

Always use __construct() in new code.

Constructor Promotion

As of PHP 8.0.0, constructor parameters may also be promoted to correspond to an object property. It is very common for constructor parameters to be assigned to a property in the constructor but otherwise not operated upon. Constructor promotion provides a short-hand for that use case. The example above could be rewritten as the following.

<?php
class Point {
    public function __construct(protected int $x, protected int $y = 0) {
    }
}

When a constructor argument includes a modifier, PHP will interpret it as both an object property and a constructor argument, and assign the argument value to the property. The constructor body may then be empty or may contain other statements. Any additional statements will be executed after the argument values have been assigned to the corresponding properties.

Not all arguments need to be promoted. It is possible to mix and match promoted and not-promoted arguments, in any order. Promoted arguments have no impact on code calling the constructor.

Note:

Using a visibility modifier (public, protected or private) is the most likely way to apply property promotion, but any other single modifier (such as readonly) will have the same effect.

Note:

Object properties may not be typed callable due to engine ambiguity that would introduce. Promoted arguments, therefore, may not be typed callable either. Any other type declaration is permitted, however.

Note:

As promoted properties are desugared to both a property as well as a function parameter, any and all naming restrictions for both properties as well as parameters apply.

Note:

Attributes placed on a promoted constructor argument will be replicated to both the property and argument. Default values on a promoted constructor argument will be replicated only to the argument and not the property.

New in initializers

As of PHP 8.1.0, objects can be used as default parameter values, static variables, and global constants, as well as in attribute arguments. Objects can also be passed to define now.

Note:

The use of a dynamic or non-string class name or an anonymous class is not allowed. The use of argument unpacking is not allowed. The use of unsupported expressions as arguments is not allowed.

<?php

// All allowed:
static $x = new Foo;

const C = new Foo;
 
function test($param = new Foo) {}
 
#[AnAttribute(new Foo)]
class Test {
    public function __construct(
        public $prop = new Foo,
    ) {}
}

// All not allowed (compile-time error):
function test(
    $a = new (CLASS_NAME_CONSTANT)(), // dynamic class name
    $b = new class {}, // anonymous class
    $c = new A(...[]), // argument unpacking
    $d = new B($abc), // unsupported constant expression
) {}
?>

Static creation methods

PHP only supports a single constructor per class. In some cases, however, it may be desirable to allow an object to be constructed in different ways with different inputs. The recommended way to do so is by using static methods as constructor wrappers.

<?php
class Product {

    private ?int $id;
    private ?string $name;

    private function __construct(?int $id = null, ?string $name = null) {
        $this->id = $id;
        $this->name = $name;
    }

    public static function fromBasicData(int $id, string $name): static {
        $new = new static($id, $name);
        return $new;
    }

    public static function fromJson(string $json): static {
        $data = json_decode($json, true);
        return new static($data['id'], $data['name']);
    }

    public static function fromXml(string $xml): static {
        // Custom logic here.
        $data = convert_xml_to_array($xml);
        $new = new static();
        $new->id = $data['id'];
        $new->name = $data['name'];
        return $new;
    }
}

$p1 = Product::fromBasicData(5, 'Widget');
$p2 = Product::fromJson($some_json_string);
$p3 = Product::fromXml($some_xml_string);

The constructor may be made private or protected to prevent it from being called externally. If so, only a static method will be able to instantiate the class. Because they are in the same class definition they have access to private methods, even if not of the same object instance. The private constructor is optional and may or may not make sense depending on the use case.

The three public static methods then demonstrate different ways of instantiating the object.

• fromBasicData() takes the exact parameters that are needed, then creates the object by calling the constructor and returning the result.
• fromJson() accepts a JSON string and does some pre-processing on it itself to convert it into the format desired by the constructor. It then returns the new object.
• fromXml() accepts an XML string, preprocesses it, and then creates a bare object. The constructor is still called, but as all of the parameters are optional the method skips them. It then assigns values to the object properties directly before returning the result.

In all three cases, the static keyword is translated into the name of the class the code is in. In this case, Product.