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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 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 with other methods, PHP will not generate an E_STRICT level error message when __construct() is overridden with different parameters than the parent __construct() method has.

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 $xint $y 0) {
        
$this->$x;
        
$this->$y;
    }
}

// Pass both parameters.
$p1 = new Point(45);
// 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(y5x4);
?>

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.

Example #3 Using constructor property promotion

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

When a constructor argument includes a visibility 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:

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:

Attributes placed on a promoted constructor argument will be replicated to both the property and argument.

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 #4 Using static creation methods

<?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 $idstring $name): static {
        
$new = new static($id$name);
        return 
$new;
    }

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

    public static function 
fromXml(string $xml): static {
        
// Put your own 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.

Destructor

__destruct ( ) : void

PHP 5 introduces 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 #5 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.

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.

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User Contributed Notes 11 notes

up
104
david dot scourfield at llynfi dot co dot uk
9 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) 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:

<?php

header
("Content-type: text/plain");

class
Foo {
   
   
/**
     * An indentifier
     * @var string
     */
   
private $name;
   
/**
     * A reference to another Foo object
     * @var Foo
     */
   
private $link;

    public function
__construct($name) {
       
$this->name = $name;
    }

    public function
setLink(Foo $link){
       
$this->link = $link;
    }

    public function
__destruct() {
        echo
'Destroying: ', $this->name, PHP_EOL;
    }
}

// create two Foo objects:
$foo = new Foo('Foo 1');
$bar = new Foo('Foo 2');

// make them point to each other
$foo->setLink($bar);
$bar->setLink($foo);

// destroy the global references to them
$foo = null;
$bar = null;

// we now have no way to access Foo 1 or Foo 2, so they OUGHT to be __destruct()ed
// but they are not, so we get a memory leak as they are still in memory.
//
// Uncomment the next line to see the difference when explicitly calling the GC:
// gc_collect_cycles();
//
// see also: http://www.php.net/manual/en/features.gc.php
//

// create two more Foo objects, but DO NOT set their internal Foo references
// so nothing except the vars $foo and $bar point to them:
$foo = new Foo('Foo 3');
$bar = new Foo('Foo 4');

// destroy the global references to them
$foo = null;
$bar = null;

// we now have no way to access Foo 3 or Foo 4 and as there are no more references
// to them anywhere, their __destruct() methods are automatically called here,
// BEFORE the next line is executed:

echo 'End of script', PHP_EOL;

?>

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.
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22
domger at freenet dot de
3 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.
up
15
spleen
12 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!
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5
bolshun at mail dot ru
12 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.
up
6
prieler at abm dot at
13 years ago
i have written a quick example about the order of destructors and shutdown functions in php 5.2.1:

<?php
class destruction {
    var
$name;

    function
destruction($name) {
       
$this->name = $name;
       
register_shutdown_function(array(&$this, "shutdown"));
    }

    function
shutdown() {
        echo
'shutdown: '.$this->name."\n";
    }

    function
__destruct() {
        echo
'destruct: '.$this->name."\n";
    }
}

$a = new destruction('a: global 1');

function
test() {
   
$b = new destruction('b: func 1');
   
$c = new destruction('c: func 2');
}
test();

$d = new destruction('d: global 2');

?>

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
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9
Per Persson
8 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[] = $row;
    }

    public function
save()
    {
        echo
'<ul>';
        foreach (
$this->rows as $row)
        {
            echo
'<li>', $row, '</li>';
        }
        echo
'</ul>';
    }
}

$logger = new Logger;
$logger->log('Before');

$nonset->foo();

$logger->log('After');
?>

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[] = $row;
    }
   
    public function
save()
    {
        echo
'<ul>';
        foreach (
$this->rows as $row)
        {
            echo
'<li>', $row, '</li>';
        }
        echo
'</ul>';
    }
}

$logger = new Logger;
$logger->log('Before');

$nonset->foo();

$logger->log('After');
?>
Now Before will be printed, but not After, so you can see that a shutdown occurred after Before.
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3
Yousef Ismaeil cliprz[At]gmail[Dot]com
7 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 $deviceName,$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 = $name;
       
$this->deviceVersion = $version;
       
$this->deviceColor = $color;
        echo
"The ".__CLASS__." class is stratup.<br /><br />";
    }
   
   
/**
     * 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
'<br /><br />Dumpping Mobile::deviceName to make sure its removed, Olay :';
       
var_dump($this->deviceName);
        echo
"<br />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.
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0
iwwp at outlook dot com
10 months ago
To better understand the __destrust method:

class A {
    protected $id;

    public function __construct($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
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1
Jonathon Hibbard
10 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->error_reporting = $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 ;)
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1
Reza Mahjourian
14 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.

<?php
class Employee {
   public function
__construct($inName) {
      
$this->name = $inName;
   }

   public static function
constructFromDom($inDom)
   {
      
$name = $inDom->name;
       return new
Employee($name);
   }

   private
$name;
}

class
HourlyEmployee extends Employee {
   public function
__construct($inName, $inHourlyRate) {
      
parent::__construct($inName);
      
$this->hourlyRate = $inHourlyRate;
   }

   public static function
constructFromDom($inDom)
   {
      
// can't call parent::constructFromDom($inDom)
       // need to do all the work here again
      
$name = $inDom->name// increased coupling
      
$hourlyRate = $inDom->hourlyrate;
       return new
EmployeeHourly($name, $hourlyRate);
   }

   private
$hourlyRate;
}
?>

The only solution is to merge the two constructors in one by adding an optional $inDom parameter to every constructor.
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0
david at synatree dot com
12 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)
    {
            $this->C = new SimpleCrypt($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 $this->C->encrypt($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.
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