Scope Resolution Operator (::)

The Scope Resolution Operator (also called Paamayim Nekudotayim) or in simpler terms, the double colon, is a token that allows access to static, constant, and overridden properties or methods of a class.

When referencing these items from outside the class definition, use the name of the class.

As of PHP 5.3.0, it's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static).

Paamayim Nekudotayim would, at first, seem like a strange choice for naming a double-colon. However, while writing the Zend Engine 0.5 (which powers PHP 3), that's what the Zend team decided to call it. It actually does mean double-colon - in Hebrew!

Example #1 :: from outside the class definition

<?php
class MyClass {
    const 
CONST_VALUE 'A constant value';
}

$classname 'MyClass';
echo 
$classname::CONST_VALUE// As of PHP 5.3.0

echo MyClass::CONST_VALUE;
?>

Three special keywords self, parent and static are used to access properties or methods from inside the class definition.

Example #2 :: from inside the class definition

<?php
class OtherClass extends MyClass
{
    public static 
$my_static 'static var';

    public static function 
doubleColon() {
        echo 
parent::CONST_VALUE "\n";
        echo 
self::$my_static "\n";
    }
}

$classname 'OtherClass';
echo 
$classname::doubleColon(); // As of PHP 5.3.0

OtherClass::doubleColon();
?>

When an extending class overrides the parents definition of a method, PHP will not call the parent's method. It's up to the extended class on whether or not the parent's method is called. This also applies to Constructors and Destructors, Overloading, and Magic method definitions.

Example #3 Calling a parent's method

<?php
class MyClass
{
    protected function 
myFunc() {
        echo 
"MyClass::myFunc()\n";
    }
}

class 
OtherClass extends MyClass
{
    
// Override parent's definition
    
public function myFunc()
    {
        
// But still call the parent function
        
parent::myFunc();
        echo 
"OtherClass::myFunc()\n";
    }
}

$class = new OtherClass();
$class->myFunc();
?>

See also some examples of static call trickery.

add a note add a note

User Contributed Notes 17 notes

up
12
richard at richard-sumilang dot com
6 years ago
Actually, for people not using PHP 5.3 yet you should try avoiding the use of a eval() at all costs! THere are too many security risks and dirty code that come from using eval. If you want to call a static method from a class then you can use call_user_func() instead which is much safer.

Example:

clas Foo{

  public static method Bar(){
    echo "Hello world!";
  }

}

And to execute that with call_user_func you would do the following:

call_user_func(array('Foo', 'bar') [, $params] );

Thanks,
- Richard S.
up
9
csaba dot dobai at php-sparcle dot com
5 years ago
For the 'late static binding' topic I published a code below, that demonstrates a trick for how to setting variable value in the late class, and print that in the parent (or the parent's parent, etc.) class.

<?php

class cA
{
   
/**
     * Test property for using direct default value
     */
   
protected static $item = 'Foo';
   
   
/**
     * Test property for using indirect default value
     */
   
protected static $other = 'cA';
   
    public static function
method()
    {
        print
self::$item."\r\n"; // It prints 'Foo' on everyway... :(
       
print self::$other."\r\n"; // We just think that, this one prints 'cA' only, but... :)
   
}
   
    public static function
setOther($val)
    {
       
self::$other = $val; // Set a value in this scope.
   
}
}

class
cB extends cA
{
   
/**
     * Test property with redefined default value
     */
   
protected static $item = 'Bar';
   
    public static function
setOther($val)
    {
       
self::$other = $val;
    }
}

class
cC extends cA
{
   
/**
     * Test property with redefined default value
     */
   
protected static $item = 'Tango';
   
    public static function
method()
    {
        print
self::$item."\r\n"; // It prints 'Foo' on everyway... :(
       
print self::$other."\r\n"; // We just think that, this one prints 'cA' only, but... :)
   
}
   
   
/**
     * Now we drop redeclaring the setOther() method, use cA with 'self::' just for fun.
     */
}

class
cD extends cA
{
   
/**
     * Test property with redefined default value
     */
   
protected static $item = 'Foxtrot';
   
   
/**
     * Now we drop redeclaring all methods to complete this issue.
     */
}

cB::setOther('cB'); // It's cB::method()!
cB::method(); // It's cA::method()!
cC::setOther('cC'); // It's cA::method()!
cC::method(); // It's cC::method()!
cD::setOther('cD'); // It's cA::method()!
cD::method(); // It's cA::method()!

/**
* Results: ->
* Foo
* cB
* Tango
* cC
* Foo
* cD
*
* What the hell?! :)
*/

?>
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9
remy dot damour at ----no-spam---laposte dot net
4 years ago
As of php 5.3.0, you can use 'static' as scope value as in below example (add flexibility to inheritance mechanism compared to 'self' keyword...)

<?php

class A {
    const
C = 'constA';
    public function
m() {
        echo static::
C;
    }
}

class
B extends A {
    const
C = 'constB';
}

$b = new B();
$b->m();

// output: constB
?>
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4
Theriault
4 years ago
A class constant, class property (static), and class function (static) can all share the same name and be accessed using the double-colon.

<?php

class A {

    public static
$B = '1'; # Static class variable.

   
const B = '2'; # Class constant.
   
   
public static function B() { # Static class function.
       
return '3';
    }
   
}

echo
A::$B . A::B . A::B(); # Outputs: 123
?>
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6
luka8088 at gmail dot com
5 years ago
Little static trick to go around php strict standards ...
Function caller founds an object from which it was called, so that static method can alter it, replacement for $this in static function but without strict warnings :)

<?php

error_reporting
(E_ALL + E_STRICT);

function
caller () {
 
$backtrace = debug_backtrace();
 
$object = isset($backtrace[0]['object']) ? $backtrace[0]['object'] : null;
 
$k = 1;
       
  while (isset(
$backtrace[$k]) && (!isset($backtrace[$k]['object']) || $object === $backtrace[$k]['object']))
   
$k++;

  return isset(
$backtrace[$k]['object']) ? $backtrace[$k]['object'] : null;
}

class
a {

  public
$data = 'Empty';
 
  function
set_data () {
   
b::set();
  }

}

class
b {

  static function
set () {
   
// $this->data = 'Data from B !';
    // using this in static function throws a warning ...
   
caller()->data = 'Data from B !';
  }

}

$a = new a();
$a->set_data();
echo
$a->data;

?>

Outputs: Data from B !

No warnings or errors !
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2
mongoose643 at gmail dot com
7 years ago
This is a solution for those that still need to write code compatible with php 4 but would like to use the flexibility of static variables. PHP 4 does not support static variables within the class scope but it does support them within the scope of class methods. The following is a bit of a workaround to store data in static mode in php 4.

Note: This code also works in PHP 5.

(Tested on version 4.3.1+)

The tricky part is when using when arrays you have to do a bit of fancy coding to get or set individual elements in the array. The example code below should show you the basics of it though.

<?php

class StaticSample
{
   
//Copyright Michael White (www.crestidg.com) 2007
    //You may use and modify this code but please keep this short copyright notice in tact.
    //If you modify the code you may comment the changes you make and append your own copyright
    //notice to mine. This code is not to be redistributed individually for sale but please use it as part
    //of your projects and applications - free or non-free.
   
   
    //Static workaround for php4 - even works with arrays - the trick is accessing the arrays.
    //I used the format s_varname for my methods that employ this workaround. That keeps it
    //similar to working with actual variables as much as possible.
    //The s_ prefix immediately identifies it as a static variable workaround method while
    //I'm looking thorugh my code.
   
function &s_foo($value=null, $remove=null)
    {
        static
$s_var;    //Declare the static variable.    The name here doesn't matter - only the name of the method matters.
       
       
if($remove)
        {
            if(
is_array($value))
            {
                if(
is_array($s_var))
                {
                    foreach(
$value as $key => $data)
                    {
                        unset(
$s_var[$key]);
                    }
                }
            }
            else
            {
               
//You can't just use unset() here because the static state of the variable will bring back the value next time you call the method.
               
$s_var = null;
                unset(
$s_var);
            }
           
//Make sure that you don't set the value over again.
           
$value = null;
        }
        if(
$value)
        {
            if(
is_array($value))
            {
                if(
is_array($s_var))
                {
                   
//$s_var = array_merge($s_var, $value);        //Doesn't overwrite values. This adds them - a property of the array_merge() function.
                   
foreach($value as $key => $data)
                    {
                       
$s_var[$key] = $data;    //Overwrites values.
                   
}
                }
                else
                {
                   
$s_var = $value;
                }
            }
            else
            {
               
$s_var = $value;
            }
        }
       
        return
$s_var;
    }
}

echo
"Working with non-array values.<br>";
echo
"Before Setting: ".StaticSample::s_foo();
echo
"<br>";
echo
"While Setting: ".StaticSample::s_foo("VALUE HERE");
echo
"<br>";
echo
"After Setting: ".StaticSample::s_foo();
echo
"<br>";
echo
"While Removing: ".StaticSample::s_foo(null, 1);
echo
"<br>";
echo
"After Removing: ".StaticSample::s_foo();
echo
"<hr>";
echo
"Working with array values<br>";
$array = array(0=>"cat", 1=>"dog", 2=>"monkey");
echo
"Set an array value: ";
print_r(StaticSample::s_foo($array));
echo
"<br>";

//Here you need to get all the values in the array then sort through or choose the one(s) you want.
$all_elements = StaticSample::s_foo();
$middle_element = $all_elements[1];
echo
"The middle element: ".$middle_element;
echo
"<br>";

$changed_array = array(1=>"big dog", 3=>"bat", "bird"=>"flamingo");
echo
"Changing the value: ";
print_r(StaticSample::s_foo($changed_array));
echo
"<br>";

//All you have to do here is create an array with the keys you want to erase in it.
//If you want to erase all keys then don't pass any array to the method.
$element_to_erase = array(3=>null);
echo
"Erasing the fourth element: ";
$elements_left = StaticSample::s_foo($element_to_erase, 1);
print_r($elements_left);
echo
"<br>";
echo
"Enjoy!";

?>
up
3
wouter at interpotential dot com
5 years ago
It's worth noting, that the mentioned variable can also be an object instance. This appears to be the easiest way to refer to a static function as high in the inheritance hierarchy as possible, as seen from the instance. I've encountered some odd behavior while using static::something() inside a non-static method.

See the following example code:

<?php
class FooClass {
    public function
testSelf() {
        return
self::t();
    }

    public function
testThis() {
        return
$this::t();
    }

    public static function
t() {
        return
'FooClass';
    }

    function
__toString() {
        return
'FooClass';
    }
}

class
BarClass extends FooClass {
    public static function
t() {
        return
'BarClass';
    }

}

$obj = new BarClass();
print_r(Array(
   
$obj->testSelf(), $obj->testThis(),
));
?>

which outputs:

<pre>
Array
(
    [0] => FooClass
    [1] => BarClass
)
</pre>

As you can see, __toString has no effect on any of this. Just in case you were wondering if perhaps this was the way it's done.
up
1
guy at syntheticwebapps dot com
1 year ago
It seems as though you can use more than the class name to reference the static variables, constants, and static functions of a class definition from outside that class using the :: . The language appears to allow you to use the object itself.

For example:
class horse
{
   static $props = {'order'=>'mammal'};
}
$animal = new horse();
echo $animal::$props['order'];

// yields 'mammal'

This does not appear to be documented but I see it as an important convenience in the language. I would like to see it documented and supported as valid.

If it weren't supported officially, the alternative would seem to be messy, something like this:

$animalClass = get_class($animal);
echo $animalClass::$props['order'];
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0
jasverix at NOSPAM dot gmail dot com
1 year ago
Just found out that using the class name may also work to call similar function of anchestor class.

<?php

class Anchestor {
  
   public
$Prefix = '';

   private
$_string 'Bar';
    public function
Foo() {
        return
$this->Prefix.$this->_string;
    }
}

class
MyParent extends Anchestor {
    public function
Foo() {
        
$this->Prefix = null;
        return
parent::Foo().'Baz';
    }
}

class
Child extends MyParent {
    public function
Foo() {
       
$this->Prefix = 'Foo';
        return
Anchestor::Foo();
    }
}

$c = new Child();
echo
$c->Foo(); //return FooBar, because Prefix, as in Anchestor::Foo()

?>

The Child class calls at Anchestor::Foo(), and therefore MyParent::Foo() is never run.
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0
erwinhaantjes at interurbia dot nl
5 years ago
Here is a nice function i like to share with you because it so nice and handy feature. I wanted to get rid off the parent::methodName(myVar1, myVar2, myVar3 etc.....) construction so i wrote a shortcut to this notation. The only thing you have to do is calling $this->inherited() (or parent::inherited()) to call the same function with the same parameters in the parent class. It is also possible to specify other parameters if you like. You can also use this inside constructors/destructors.

To use this example, put it in the base class of all objects.You also need the simple function below to test if an array is valid (from a self-build library) but you are free to change it to your own needs ;-):

<?php
function suIsValidArray( &$a, &$iCount = null )
{
 
$iCount = 0;
  return (
is_array( $a ) and (( $iCount = @count( $a )) > 0 )); 
}
?>

The inherited function/construction is familiar language construction in Object Pascal/Delphi (wonderful language). If you have any comments, please enter a note.  

Here it is:

<?php
   
public function inherited()
    {
      
// Use DEBUG backtrace to trace caller function
     
$bt = debug_backtrace();
     
$bt = $bt[ 1 ]; // List is in reversed order, 0 reffers to this function so get previous one
     
     
if( !suIsValidArray( $bt ))
      { return; }
     
     
$sClassName = $bt["class"];
     
$sParentClassName = get_parent_class( $sClassName );
      if( empty(
$sClassName ) or empty( $sParentClassName ) or $sParentClassName == $sClassName )
      { return; }
     
     
$sFuncName = $bt["function"];
      
// constructor or destructor called (old fashion way)?
     
if( ( $bIsConstruct = ( $sFuncName == $sClassName )) or ( $sFuncName == "_".$sClassName ) )
      {
         
// get parent constructor/destructor
         
$sFuncName = (( !$bIsConstruct ) ? "_" : "" ).$sParentClassName;
        if(
$sFuncName == (( !$bIsConstruct ) ? "_" : "" ).$sClassName )
        { return; }
      }     
      
      if(
method_exists( $sParentClassName, $sFuncName ))
       {
            
// If there are parameters specified, use these
        
$args = func_get_args();
         if( !
suIsValidArray( $args ))
          {
$args = &$bt["args"]; } // otherwise use previous function parameters if any
     
          
$iCount = 0;
          
$sArgs = "";
        if(
suIsValidArray( $args, $iCount ))
         {
           for(
$i = 0; $i < $iCount; $i++ )
            {
$sArgs.="&$"."args[$i]".( $i < ($iCount-1) ? "," : "" ); }
         }   
        
        
// Simple, evaluate it, this is possible because it is done inside the class itself, and
         // the parent class is already created because is a part of the class structure, no worry, no scope issues  
       
eval( "$"."result = $sParentClassName::".$sFuncName."($sArgs);" );
        return @
$result;
       } 
    }
?>
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0
Kristof Coomans
8 years ago
In response to ian at [first name]henderson dot org:

(related bogus bug report: http://bugs.php.net/bug.php?id=26930)

The functionality you've expected maybe will be possible in PHP6, probably by using the static keyword in conjunction with the scope resolution parameter. You can read more about this in the minutes of the PHP developers meeting at 11 and 12 november in Paris: http://www.php.net/~derick/meeting-notes.html point 5.4: Late static binding using "this" without "$" (or perhaps with a different name)
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0
HuugjeWeg
9 years ago
In response to ian at [first name]henderson dot org:

You are not allowed to redefine static methods, see
http://www.php.net/manual/en/language.oop5.static.php

And in response to thenewparadigm at hotmail dot com: the behaviour you describe seems appropriate for *classes* with static variables, see "Using static variables" on http://nl2.php.net/static
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0
thenewparadigm at hotmail dot com
9 years ago
There is also a quirk with using the scope resolution operator on static class variables.  Below is an example using a highly modified version of Ian's code:

<?php

class ExampleSuperclass
{
   static
$className;

   static function
showClassName() {
      echo
self::$className . "\n";
   }
}

class
ExampleSubclassOne extends ExampleSuperclass
{
   static function
setClassName()
   {
      
self::$className = "subclassOne";
   }
}

class
ExampleSubclassTwo extends ExampleSuperClass
{
   static function
setClassName()
   {
     
self::$className = "subclassTwo";
   }
}

// setting variables for each class
ExampleSubclassOne::setClassName();
ExampleSubclassTwo::setClassName();

ExampleSubclassOne::showClassName();  // output is "subclassTwo"!

// more output:

echo ExampleSubclassOne::$className . "\n"; // output is "subclassTwo"!
echo ExampleSubclassTwo::$className . "\n"; // output is "subclassTwo"
echo ExampleSuperclass::$className . "\n"; // output is "subclassTwo"!

?>

appearantly, any static variables defined in a superclass are directly referenced in subclasses,
and all changes are visible throughout the class heirarchy.  care must be taken when using static
class variables.
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0
ian at [first name]henderson dot org
9 years ago
Please note that methods called by the scope resolution operator which are defined by a superclass of the first operand are called in the scope of the SUPERCLASS.  For example,

<?php

class ExampleSuperclass
{
    static function
classType()
    {
        return
"superclass";
    }

    static function
doSomething()
    {
        echo
"doing something with " . self::classType();
    }
}

class
ExampleClass extends ExampleSuperclass
{
    static function
classType()
    {
        return
"subclass";
    }
}

ExampleClass::doSomething();
// output is "doing something with superclass"!

?>

This can be surprising (it surprised me!) when coming from other object-oriented languages, which would output "doing something with subclass" in this case.
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-1
barss dot dev at gmail dot com
6 years ago
Nice trick with scope resolution
<?php
   
class A
   
{
        public function
TestFunc()
        {
            return
$this->test;
        }
    }

    class
B
   
{
        public
$test;

        public function
__construct()
        {
           
$this->test = "Nice trick";
        }

        public function
GetTest()
        {
            return
A::TestFunc();
        }
    }

   
$b = new B;
    echo
$b->GetTest();
?>

will output

Nice trick
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-1
developit at mail dot ru
8 years ago
You use 'self' to access this class, 'parent' - to access parent class, and what will you do to access a parent of the parent? Or to access the very root class of deep class hierarchy? The answer is to use classnames. That'll work just like 'parent'. Here's an example to explain what I mean. Following code

<?php
class A
{
    protected
$x = 'A';
    public function
f()
    {
        return
'['.$this->x.']';
    }
}

class
B extends A
{
    protected
$x = 'B';
    public function
f()
    {
        return
'{'.$this->x.'}';
    }
}

class
C extends B
{
    protected
$x = 'C';
    public function
f()
    {
        return
'('.$this->x.')'.parent::f().B::f().A::f();
    }
}

$a = new A();
$b = new B();
$c = new C();

print
$a->f().'<br/>';
print
$b->f().'<br/>';
print
$c->f().'<br/>';
?>

will output

[A] -- {B} -- (C){C}{C}[C]
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-1
giovanni at gargani dot it
5 years ago
Well, a "swiss knife" couple of code lines to call parent method. The only limit is you can't use it with "by reference" parameters.
Main advantage you dont need to know the "actual" signature of your super class, you just need to know which arguments do you need

<?php
class someclass extends some superclass {
// usable for constructors
function __construct($ineedthisone) {
 
$args=func_get_args();
 
/* $args will contain any argument passed to __construct.  
  * Your formal argument doesnt influence the way func_get_args() works
  */
 
call_user_func_array(array('parent',__FUNCTION__),$args);
}
// but this is not for __construct only
function anyMethod() {
 
$args=func_get_args();
 
call_user_func_array(array('parent',__FUNCTION__),$args);
}
 
// Note: php 5.3.0 will even let you do
function anyMethod() {
 
//Needs php >=5.3.x
 
call_user_func_array(array('parent',__FUNCTION__),func_get_args());
}

}
?>
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