Constructor
PHP allows developers to declare constructor methods for classes.
Classes which have a constructor method call this method on each
newly-created object, so it is suitable for any initialization that the
object may need before it is used.
Note:
Parent constructors are not called implicitly if the child class defines
a constructor. In order to run a parent constructor, a call to
parent::__construct() within the child constructor is
required. If the child does not define a constructor then it may be inherited
from the parent class just like a normal class method (if it was not declared
as private).
Example #1 Constructors in inheritance
<?php
class BaseClass {
function __construct() {
print "In BaseClass constructor\n";
}
}
class SubClass extends BaseClass {
function __construct() {
parent::__construct();
print "In SubClass constructor\n";
}
}
class OtherSubClass extends BaseClass {
// inherits BaseClass's constructor
}
// In BaseClass constructor
$obj = new BaseClass();
// In BaseClass constructor
// In SubClass constructor
$obj = new SubClass();
// In BaseClass constructor
$obj = new OtherSubClass();
?>
Unlike other methods, __construct()
is exempt from the usual
signature compatibility rules
when being extended.
Constructors are ordinary methods which are called during the instantiation of their
corresponding object. As such, they may define an arbitrary number of arguments, which
may be required, may have a type, and may have a default value. Constructor arguments
are called by placing the arguments in parentheses after the class name.
Example #2 Using constructor arguments
<?php
class Point {
protected int $x;
protected int $y;
public function __construct(int $x, int $y = 0) {
$this->x = $x;
$this->y = $y;
}
}
// Pass both parameters.
$p1 = new Point(4, 5);
// Pass only the required parameter. $y will take its default value of 0.
$p2 = new Point(4);
// With named parameters (as of PHP 8.0):
$p3 = new Point(y: 5, x: 4);
?>
If a class has no constructor, or the constructor has no required arguments, the parentheses
may be omitted.
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.
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.
Example #4 Using new in initializers
<?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.
Example #5 Using static creation methods
<?php
$some_json_string = '{ "id": 1004, "name": "Elephpant" }';
$some_xml_string = "<animal><id>1005</id><name>Elephpant</name></animal>";
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 {
$data = simplexml_load_string($xml);
$new = new static();
$new->id = (int) $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);
var_dump($p1, $p2, $p3);
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.
Destructor
PHP possesses a destructor concept similar to that of other
object-oriented languages, such as C++. The destructor method will be
called as soon as there are no other references to a particular object,
or in any order during the shutdown sequence.
Example #6 Destructor Example
<?php
class MyDestructableClass
{
function __construct() {
print "In constructor\n";
}
function __destruct() {
print "Destroying " . __CLASS__ . "\n";
}
}
$obj = new MyDestructableClass();
Like constructors, parent destructors will not be called implicitly by
the engine. In order to run a parent destructor, one would have to
explicitly call parent::__destruct() in the destructor
body. Also like constructors, a child class may inherit the parent's
destructor if it does not implement one itself.
The destructor will be called even if script execution is stopped using
exit(). Calling exit() in a destructor
will prevent the remaining shutdown routines from executing.
If a destructor creates new references to its object, it will not be called
a second time when the reference count reaches zero again or during the
shutdown sequence.
As of PHP 8.4.0, when
cycle collection
occurs during the execution of a
Fiber, the destructors of objects
scheduled for collection are executed in a separate Fiber, called the
gc_destructor_fiber.
If this Fiber is suspended, a new one will be created to execute any
remaining destructors.
The previous gc_destructor_fiber will no longer be
referenced by the garbage collector and may be collected if it is not
referenced elsewhere.
Objects whose destructor are suspended will not be collected until the
destructor returns or the Fiber itself is collected.
Note:
Destructors called during the script shutdown have HTTP headers already
sent. The working directory in the script shutdown phase can be different
with some SAPIs (e.g. Apache).
Note:
Attempting to throw an exception from a destructor (called in the time of
script termination) causes a fatal error.
