Constructors and Destructors

Constructor
void
__construct
mixed
values
""
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.
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).
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.
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.
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.
Using constructor property promotion
<?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.
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.
Object properties may not be typed 
callable due to engine ambiguity that this would introduce. Promoted arguments, therefore, may not be typed 
callable either. Any other type declaration is permitted, however.
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.
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.
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.
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.
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
void
__destruct
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.
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 is suspended will not be collected until the destructor returns or the Fiber itself is collected.
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).
Attempting to throw an exception from a destructor (called in the time of script termination) causes a fatal error.