PHP: Hypertext Preprocessor (original) (raw)

Constructors and Destructors

Constructor

__construct(mixed ...$values = ""): void

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 toparent::__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 usualsignature 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,intx, int x,inty = 0) { this−>x=this->x = this−>x=x; this−>y=this->y = 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](errorfunc.constants.php#constant.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=newB(d = new B(d=newB(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 = "1005Elephpant";

class

Product {

private ?

int $id;
private ?string $name;

private function

__construct(?int id=null,?stringid = null, ?string id=null,?stringname = null) { this−>id=this->id = this−>id=id; this−>name=this->name = this−>name=name;
}

public static function

fromBasicData(int id,stringid, string id,stringname): static { new=newstatic(new = new static(new=newstatic(id, $name);
return $new;
}

public static function

fromJson(string $json): static { data=jsondecode(data = json_decode(data=jsond​ecode(json, true);
return new static($data['id'], $data['name']);
}

public static function

fromXml(string $xml): static {a
data=simplexmlloadstring(data = simplexml_load_string(data=simplexmll​oads​tring(xml);
$new = new static(); new−>id=new->id = new−>id=data->id; new−>name=new->name = new−>name=data->name;
return $new;
}
}$p1 = Product::fromBasicData(5, 'Widget'); p2=Product::fromJson(p2 = Product::fromJson(p2=Product::fromJson(some_json_string); p3=Product::fromXml(p3 = Product::fromXml(p3=Product::fromXml(some_xml_string);var_dump($p1, p2,p2, 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

__destruct(): void

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

`<?phpclass 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 usingexit(). 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, whencycle collection occurs during the execution of aFiber, the destructors of objects scheduled for collection are executed in a separate Fiber, called thegc_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.

Found A Problem?

david dot scourfield at llynfi dot co dot uk ¶

13 years ago

`Be aware of potential memory leaks caused by circular references within objects. The PHP manual states "[t]he destructor method will be called as soon as all references to a particular object are removed" and this is precisely true: if two objects reference each other (or even if one object has a field that points to itself as in this−>foo=this->foo = this−>foo=this) then this reference will prevent the destructor being called even when there are no other references to the object at all. The programmer can no longer access the objects, but they still stay in memory.

Consider the following example:

This will output:

Destroying: Foo 3
Destroying: Foo 4
End of script
Destroying: Foo 1
Destroying: Foo 2

But if we uncomment the gc_collect_cycles(); function call in the middle of the script, we get:

Destroying: Foo 2
Destroying: Foo 1
Destroying: Foo 3
Destroying: Foo 4
End of script

As may be desired.

NOTE: calling gc_collect_cycles() does have a speed overhead, so only use it if you feel you need to.

`

Hayley Watson ¶

1 year ago

`There are other advantages to using static factory methods to wrap object construction instead of bare constructor calls.

As well as allowing for different methods to use in different scenarios, with more relevant names both for the methods and the parameters and without the constructor having to handle different sets of arguments of different types:

• You can do all your input validation before attempting to construct the object.
• The object itself can bypass that input validation when constructing new instances of its own class, since you can ensure that it knows what it's doing.
• With input validation/preprocessing moved to the factory methods, the constructor itself can often be reduced to "set these properties to these arguments", meaning the constructor promotion syntax becomes more useful.
• Having been hidden away from users, the constructor's signature can be a bit uglier without becoming a pain for them. Heh.
• Static methods can be lifted and passed around as first class closures, to be called in the normal fashion wherever functions can be called, without the special "new" syntax.
• The factory method need not return a new instance of that exact class. It could return a pre-existing instance that would do the same job as the new one would (especially useful in the case of immutable "value type" objects by reducing duplication); or a simpler or more specific subclass to do the job with less overhead than a more generic instance of the original class. Returning a subclass means LSP still holds.

`

domger at freenet dot de ¶

7 years ago

`The __destruct magic method must be public.

public function __destruct()
{
;
}

The method will automatically be called externally to the instance. Declaring __destruct as protected or private will result in a warning and the magic method will not be called.

Note: In PHP 5.3.10 i saw strange side effects while some Destructors were declared as protected.

`

spleen ¶

16 years ago

`It's always the easy things that get you -

Being new to OOP, it took me quite a while to figure out that there are TWO underscores in front of the word __construct.

It is __construct
Not _construct

Extremely obvious once you figure it out, but it can be sooo frustrating until you do.

I spent quite a bit of needless time debugging working code.

I even thought about it a few times, thinking it looked a little long in the examples, but at the time that just seemed silly(always thinking "oh somebody would have made that clear if it weren't just a regular underscore...")

All the manuals I looked at, all the tuturials I read, all the examples I browsed through - not once did anybody mention this!

(please don't tell me it's explained somewhere on this page and I just missed it, you'll only add to my pain.)

I hope this helps somebody else!

`

iwwp at outlook dot com ¶

5 years ago

`To better understand the __destrust method:

class A {
protected $id;

public function __construct($id)
{
this−>id=this->id = this−>id=id;
echo "construct {$this->id}\n";
}

public function __destruct()
{
echo "destruct {$this->id}\n";
}
}

$a = new A(1);
echo "-------------\n";
$aa = new A(2);
echo "=============\n";

The output content:

construct 1

construct 2

destruct 2
destruct 1

`

mmulej at gmail dot com ¶

3 years ago

`` I can't edit my previous note to elaborate on modifiers. Please excuse me.

If both parent and child classes have a method with the same name defined, and it is called in parent's constructor, using parent::__construct() will call the method in the child.

In this example both A::method and C::method are public.

You may change A::method to protected, and C::method to protected or public and it will still work the same.

If however you set A::method as private, it doesn't matter whether C::method is private, protected or public. Both bandb and bandc will echo 'A'.

``

david at synatree dot com ¶

17 years ago

`When a script is in the process of die()ing, you can't count on the order in which __destruct() will be called.

For a script I have been working on, I wanted to do transparent low-level encryption of any outgoing data. To accomplish this, I used a global singleton class configured like this:

class EncryptedComms
{
private $C;
private $objs = array();
private static $_me;

public static function destroyAfter(&$obj)
{
self::getInstance()->objs[] =& $obj;
/*
Hopefully by forcing a reference to another object to exist
inside this class, the referenced object will need to be destroyed
before garbage collection can occur on this object. This will force
this object's destruct method to be fired AFTER the destructors of
all the objects referenced here.
*/
}
public function __construct($key)
{
this−>C=newSimpleCrypt(this->C = new SimpleCrypt(this−>C=newSimpleCrypt(key);
ob_start(array($this,'getBuffer'));
}
public static function &getInstance($key=NULL)
{
if(!self::$_me && $key)
self::$_me = new EncryptedComms($key);
else
return self::$_me;
}

public function __destruct()
{
ob_end_flush();
}

public function getBuffer($str)
{
return this−>C−>encrypt(this->C->encrypt(this−>C−>encrypt(str);
}

}

In this example, I tried to register other objects to always be destroyed just before this object. Like this:

class A
{

public function __construct()
{
EncryptedComms::destroyAfter($this);
}
}

One would think that the references to the objects contained in the singleton would be destroyed first, but this is not the case. In fact, this won't work even if you reverse the paradigm and store a reference to EncryptedComms in every object you'd like to be destroyed before it.

In short, when a script die()s, there doesn't seem to be any way to predict the order in which the destructors will fire.

`

Per Persson ¶

12 years ago

`As of PHP 5.3.10 destructors are not run on shutdown caused by fatal errors.

For example:
<?php
class Logger
{
protected $rows = array();

public function

__destruct()
{
$this->save();
}

public function

log($row)
{ this−>rows[]=this->rows[] = this−>rows[]=row;
}

public function

save()
{
echo '

';
foreach ($this->rows as $row)
{
echo '
• ', $row, '
';
}
echo '

Without the $nonset->foo(); line, Before and After will both be printed, but with the line neither will be printed.

One can however register the destructor or another method as a shutdown function:
<?php
class Logger
{
protected $rows = array();

public function

__construct()
{
register_shutdown_function(array($this, '__destruct'));
}

public function

__destruct()
{
$this->save();
}

public function

log($row)
{ this−>rows[]=this->rows[] = this−>rows[]=row;
}

public function

save()
{
echo '

';
foreach ($this->rows as $row)
{
echo '
• ', $row, '
';
}
echo '

`

prieler at abm dot at ¶

17 years ago

`i have written a quick example about the order of destructors and shutdown functions in php 5.2.1:

this will output:
shutdown: a: global 1
shutdown: b: func 1
shutdown: c: func 2
shutdown: d: global 2
destruct: b: func 1
destruct: c: func 2
destruct: d: global 2
destruct: a: global 1

conclusions:
destructors are always called on script end.
destructors are called in order of their "context": first functions, then global objects
objects in function context are deleted in order as they are set (older objects first).
objects in global context are deleted in reverse order (older objects last)

shutdown functions are called before the destructors.
shutdown functions are called in there "register" order. ;)

regards, J

`

Yousef Ismaeil cliprz[At]gmail[Dot]com ¶

11 years ago

`<?php/**

• a funny example Mobile class
• @author Yousef Ismaeil Cliprz[At]gmail[Dot]com
  /class Mobile {/*
• Some device properties
• @var string
• @access public
  /
  public deviceName,deviceName,deviceName,deviceVersion,$deviceColor;/*
• Set some values for Mobile::properties
• @param string device name
• @param string device version
• @param string device color
  /
  public function __construct ($name,$version,$color) { this−>deviceName=this->deviceName = this−>deviceName=name; this−>deviceVersion=this->deviceVersion = this−>deviceVersion=version; this−>deviceColor=this->deviceColor = this−>deviceColor=color;
  echo "The ".CLASS." class is stratup.
  
  ";
  }/*
• Some Output
• @access public
  /
  public function printOut () {
  echo 'I have a '.$this->deviceName
  .' version '.$this->deviceVersion
  .' my device color is : '.$this->deviceColor;
  }/*
• Umm only for example we will remove Mobile::$deviceName Hum not unset only to check how __destruct working
• @access public
  */
  public function __destruct () {
  $this->deviceName = 'Removed';
  echo '
  
  Dumpping Mobile::deviceName to make sure its removed, Olay :';
  var_dump($this->deviceName);
  echo "
  The ".CLASS." class is shutdown.";
  }

}

// Oh ya instance
$mob = new Mobile('iPhone','5','Black');// print output
$mob->printOut();?>

The Mobile class is stratup.

I have a iPhone version 5 my device color is : Black

Dumpping Mobile::deviceName to make sure its removed, Olay :
string 'Removed' (length=7)

The Mobile class is shutdown.

`

bolshun at mail dot ru ¶

17 years ago

Ensuring that instance of some class will be available in destructor of some other class is easy: just keep a reference to that instance in this other class.

Jonathon Hibbard ¶

15 years ago

`Please be aware of when using __destruct() in which you are unsetting variables...

Consider the following code:
<?php
class my_class {
public $error_reporting = false;

function

__construct($error_reporting = false) { this−>errorreporting=this->error_reporting = this−>errorr​eporting=error_reporting;
}

function

__destruct() {
if($this->error_reporting === true) $this->show_report();
unset($this->error_reporting);
}
?>

The above will result in an error:
Notice: Undefined property: my_class::$error_reporting in my_class.php on line 10

It appears as though the variable will be unset BEFORE it actually can execute the if statement. Removing the unset will fix this. It's not needed anyways as PHP will release everything anyways, but just in case you run across this, you know why ;)

`

Reza Mahjourian ¶

18 years ago

`Peter has suggested using static methods to compensate for unavailability of multiple constructors in PHP. This works fine for most purposes, but if you have a class hierarchy and want to delegate parts of initialization to the parent class, you can no longer use this scheme. It is because unlike constructors, in a static method you need to do the instantiation yourself. So if you call the parent static method, you will get an object of parent type which you can't continue to initialize with derived class fields.

Imagine you have an Employee class and a derived HourlyEmployee class and you want to be able to construct these objects out of some XML input too.

The only solution is to merge the two constructors in one by adding an optional $inDom parameter to every constructor.

`

ziggy at start dot dust ¶

2 years ago

`Please note that constructor argument promotion is kind of half-baked (at least as of 8.1 and it does not look to be changed in 8.2) and you are not allowed to reuse promoted argument with other promoted arguments. For example having "old style" constructor:

you will not be able to use argument promotion like this:

nor

as in both cases you will face "PHP Fatal error: Constant expression contains invalid operations".

`