静态（static）关键字

Tip

本页说明了用 static 关键字来定义静态方法和属性。static 也可用于定义静态变量以及后期静态绑定。参见上述页面了解 static 在其中的用法。

声明类属性或方法为静态，就可以不实例化类而直接访问。静态属性不能通过一个类已实例化的对象来访问（但静态方法可以）。

静态方法

由于静态方法不需要通过对象即可调用，所以伪变量 $this 在静态方法中不可用。

Warning

用静态方式调用一个非静态方法会抛出 Error。

在 PHP 8.0.0 之前，通过静态方式调用一个非静态方法这种用法已经被废弃，并且会导致一个 E_DEPRECATED 级别的警告。

Example #1 静态方法示例

<?php
class Foo {
    public static function aStaticMethod() {
        // ...
    }
}

Foo::aStaticMethod();
$classname = 'Foo';
$classname::aStaticMethod();
?>

静态属性

静态属性不可以由对象通过 -> 操作符来访问。

就像其它所有的 PHP 静态变量一样，静态属性在初始化时遵循和 const 表达式一样的规则： 有一些特定的表达式是可行的，取决于它们是否能在编译时就计算出值。

通过变量来引用一个类是可行的，但这个变量的值不能是一个保留字 （例如self，parent和 static）

Example #2 静态属性示例

<?php
class Foo
{
    public static $my_static = 'foo';

    public function staticValue() {
        return self::$my_static;
    }
}

class Bar extends Foo
{
    public function fooStatic() {
        return parent::$my_static;
    }
}


print Foo::$my_static . "\n";

$foo = new Foo();
print $foo->staticValue() . "\n";
print $foo->my_static . "\n";      // 未定义的 "属性" my_static

print $foo::$my_static . "\n";
$classname = 'Foo';
print $classname::$my_static . "\n";

print Bar::$my_static . "\n";
$bar = new Bar();
print $bar->fooStatic() . "\n";
?>

User Contributed Notes

admin at shopinson dot com 19-Aug-2020 01:20
I used instantiation to access the access the a static property directly.

A Simple ticky art, you may apply (using object to access static property in a class) with the scope resolution operator
<?php
class Shopinson {
    const MY_CONSTANT = 'the value of MY_CONSTANT ';
}

class Godwin extends Shopinson
{
    public static $myconstant = ' The Paamayim Nekudotayim or double-colon.';
    public function SaySomething(){
        echo parent::MY_CONSTANT .PHP_EOL; // outputs: the value of MY_CONSTANT
        echo self::$myconstant; // outputs: The Paamayim Nekudotayim or double-colon.
    }
}

$my_class = new Godwin();
print $my_class::$myconstant;
$my_class::SaySomething();
echo Godwin::$myconstant;
Godwin::SaySomething();

?>

print $my_class::$myconstant;
fakhar_anwar123 at hotmail dot com 26-Apr-2020 11:21
Asnwer selcted as correct solves problem. There is a valid use case (Design Pattern) where class with static member function needs to call non-static member function and before that this static members should also instantiate singleton using constructor a constructor.

**Case:**
For example, I am implementing Swoole HTTP Request event providing it a call-back as a Class with static member. Static Member does two things; it creates Singleton Object of the class by doing initialization in class constructor, and second this static members does is to call a non-static method 'run()' to handle Request (by bridging with Phalcon). Hence, static class without constructor and non-static call will not work for me.
ASchmidt at Anamera dot net 04-Sep-2018 07:18
It is important to understand the behavior of static properties in the context of class inheritance:

- Static properties defined in both parent and child classes will hold DISTINCT values for each class. Proper use of self:: vs. static:: are crucial inside of child methods to reference the intended static property.

- Static properties defined ONLY in the parent class will share a COMMON value.

<?php
declare(strict_types=1);

class staticparent {
    static    $parent_only;
    static    $both_distinct;
   
    function __construct() {
        static::$parent_only = 'fromparent';
        static::$both_distinct = 'fromparent';
    }
}

class staticchild extends staticparent {
    static    $child_only;
    static    $both_distinct;
   
    function __construct() {
        static::$parent_only = 'fromchild';
        static::$both_distinct = 'fromchild';
        static::$child_only = 'fromchild';
    }
}

$a = new staticparent;
$a = new staticchild;

echo 'Parent: parent_only=', staticparent::$parent_only, ', both_distinct=', staticparent::$both_distinct, "<br/>\r\n";
echo 'Child:  parent_only=', staticchild::$parent_only, ', both_distinct=', staticchild::$both_distinct, ', child_only=', staticchild::$child_only, "<br/>\r\n";
?>

will output:
Parent: parent_only=fromchild, both_distinct=fromparent
Child: parent_only=fromchild, both_distinct=fromchild, child_only=fromchild
vinayak dot anivase at gmail dot com 30-Mar-2018 06:38
This is also possible:

class Foo {
  public static $bar = 'a static property';
}

$baz = (new Foo)::$bar;
echo $baz;
artekpuck at gmail dot com 19-Mar-2018 05:36
It is worth mentioning that there is only one value for each static variable that is the same for all instances
b1tchcakes 27-Feb-2016 09:06
<?php

trait t {
  protected $p;
  public function testMe() {echo 'static:'.static::class. ' // self:'.self::class ."\n";}
}

class a { use t; }
class b extends a {}

echo (new a)->testMe();
echo (new b)->testMe();

outputs
static:a // self:t
static:b // self:t
rahul dot anand77 at gmail dot com 13-Nov-2015 10:39
To check if a method declared in a class is static or not, you can us following code. PHP5 has a Reflection Class, which is very helpful.

try {
    $method = new ReflectionMethod( 'className::methodName );
    if ( $method->isStatic() )
    {
        // Method is static.
    }
}
catch ( ReflectionException $e )
{
    //    method does not exist
    echo $e->getMessage();
}

*You can read more about Reflection class on http://php.net/manual/en/class.reflectionclass.php
sideshowAnthony at googlemail dot com 02-Oct-2015 07:44
The static keyword can still be used (in a non-oop way) inside a function. So if you need a value stored with your class, but it is very function specific, you can use this:

class aclass {
    public static function b(){
        static $d=12; // Set to 12 on first function call only
        $d+=12;
        return "$d\n";
    }
}

echo aclass::b(); //24
echo aclass::b(); //36
echo aclass::b(); //48
echo aclass::$d; //fatal error
jkenigso at utk dot edu 22-Dec-2013 12:20
It bears mention that static variables (in the following sense) persist:

<?php
class StaticVars
{
  public static $a=1;
}
$b=new StaticVars;
$c=new StaticVars;

echo $b::$a; //outputs 1
$c::$a=2;
echo $b::$a; //outputs 2!
?>

Note that $c::$a=2 changed the value of $b::$a even though $b and $c are totally different objects.
manishpatel2280 at gmail dot com 19-Nov-2013 02:45
In real world, we can say will use static method when we dont want to create object instance.

e.g ...

validateEmail($email) {
 if(T) return true;
return false;
}

//This makes not much sense
$obj = new Validate();
$result = $obj->validateEmail($email);

//This makes more sense
$result = Validate::validateEmail($email);
Anonymous 24-Oct-2013 01:34
It should be noted that in 'Example #2', you can also call a variably defined static method as follows:

<?php
class Foo {
    public static function aStaticMethod() {
        // ...
    }
}

$classname = 'Foo';
$methodname = 'aStaticMethod';
$classname::{$methodname}(); // As of PHP 5.3.0 I believe
?>
payal001 at gmail dot com 09-Jul-2011 03:17
Here statically accessed property prefer property of the class for which it is called. Where as self keyword enforces use of current class only. Refer the below example:

<?php
class a{

static protected $test="class a";

public function static_test(){

echo static::$test; // Results class b
echo self::$test; // Results class a

}

}

class b extends a{

static protected $test="class b";

}

$obj = new b();
$obj->static_test();
?>
gratcypalma at gmail dot om 18-Apr-2011 11:50
<?php
class foo {
    private static $getInitial;

    public static function getInitial() {
        if (self::$getInitial == null)
            self::$getInitial = new foo();
        return self::$getInitial;
    }
}

foo::getInitial();

/*
this is the example to use new class with static method..
i hope it help
*/

?>
tolean_dj at yahoo dot com 11-Dec-2010 05:09
Starting with php 5.3 you can get use of new features of static keyword. Here's an example of abstract singleton class:

<?php

abstract class Singleton {

    protected static $_instance = NULL;

    /**
     * Prevent direct object creation
     */
    final private function  __construct() { }

    /**
     * Prevent object cloning
     */
    final private function  __clone() { }

    /**
     * Returns new or existing Singleton instance
     * @return Singleton
     */
    final public static function getInstance(){
        if(null !== static::$_instance){
            return static::$_instance;
        }
        static::$_instance = new static();
        return static::$_instance;
    }
   
}
?>
Mirco 23-Aug-2010 10:16
The simplest static constructor.

Because php does not have a static constructor and you may want to initialize static class vars, there is one easy way, just call your own function directly after the class definition.

for example.

<?php
function Demonstration()
{
    return 'This is the result of demonstration()';
}

class MyStaticClass
{
    //public static $MyStaticVar = Demonstration(); //!!! FAILS: syntax error
    public static $MyStaticVar = null;

    public static function MyStaticInit()
    {
        //this is the static constructor
        //because in a function, everything is allowed, including initializing using other functions
       
        self::$MyStaticVar = Demonstration();
    }
} MyStaticClass::MyStaticInit(); //Call the static constructor

echo MyStaticClass::$MyStaticVar;
//This is the result of demonstration()
?>
webmaster at removethis dot weird-webdesign dot de 25-Feb-2010 01:38
On PHP 5.2.x or previous you might run into problems initializing static variables in subclasses due to the lack of late static binding:

<?php
class A {
    protected static $a;
   
    public static function init($value) { self::$a = $value; }
    public static function getA() { return self::$a; }
}

class B extends A {
    protected static $a; // redefine $a for own use
   
    // inherit the init() method
    public static function getA() { return self::$a; }
}

B::init('lala');
echo 'A::$a = '.A::getA().'; B::$a = '.B::getA();
?>

This will output:
A::$a = lala; B::$a =

If the init() method looks the same for (almost) all subclasses there should be no need to implement init() in every subclass and by that producing redundant code.

Solution 1:
Turn everything into non-static. BUT: This would produce redundant data on every object of the class.

Solution 2:
Turn static $a on class A into an array, use classnames of subclasses as indeces. By doing so you also don't have to redefine $a for the subclasses and the superclass' $a can be private.

Short example on a DataRecord class without error checking:

<?php
abstract class DataRecord {
    private static $db; // MySQLi-Connection, same for all subclasses
    private static $table = array(); // Array of tables for subclasses
   
    public static function init($classname, $table, $db = false) {
        if (!($db === false)) self::$db = $db;
        self::$table[$classname] = $table;
    }
   
    public static function getDB() { return self::$db; }
    public static function getTable($classname) { return self::$table[$classname]; }
}

class UserDataRecord extends DataRecord {
    public static function fetchFromDB() {
        $result = parent::getDB()->query('select * from '.parent::getTable('UserDataRecord').';');
       
        // and so on ...
        return $result; // An array of UserDataRecord objects
    }
}

$db = new MySQLi(...);
UserDataRecord::init('UserDataRecord', 'users', $db);
$users = UserDataRecord::fetchFromDB();
?>

I hope this helps some people who need to operate on PHP 5.2.x servers for some reason. Late static binding, of course, makes this workaround obsolete.
valentin at balt dot name 26-Jan-2010 07:46
How to implement a one storage place based on static properties.

<?php
class a {
   
    public function get () {
        echo $this->connect();
    }
}
class b extends a {
    private static $a;

    public function connect() {
        return self::$a = 'b';
    }
}
class c extends a {
    private static $a;

    public function connect() {
        return self::$a = 'c';
    }
}
$b = new b ();
$c = new c ();

$b->get();
$c->get();
?>
Jay Cain 18-Dec-2009 02:45
Regarding the initialization of complex static variables in a class, you can emulate a static constructor by creating a static function named something like init() and calling it immediately after the class definition.

<?php
class Example {
    private static $a = "Hello";
    private static $b;

    public static function init() {
        self::$b = self::$a . " World!";
    }
}
Example::init();
?>
davidn at xnet dot co dot nz 17-Mar-2009 04:56
Static variables are shared between sub classes

<?php
class MyParent {
   
    protected static $variable;
}

class Child1 extends MyParent {
   
    function set() {
       
        self::$variable = 2;
    }
}

class Child2 extends MyParent {
   
    function show() {
       
        echo(self::$variable);
    }
}

$c1 = new Child1();
$c1->set();
$c2 = new Child2();
$c2->show(); // prints 2
?>
zerocool at gameinsde dot ru 20-Oct-2008 01:06
Hi, here's my simple Singleton example, i think it can be useful for someone. You can use this pattern to connect to the database for example.

<?php

  class MySingleton
  {
    private static $instance = null;

    private function __construct()
    {
      $this-> name = 'Freddy';

    }

    public static function getInstance()
    {
      if(self::$instance == null)
      {
        print "Object created!<br>";
        self::$instance = new self;

      }

      return self::$instance;

    }

    public function sayHello()
    {
      print "Hello my name is {$this-> name}!<br>";

    }

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

    }

  }

  //

  $objA = MySingleton::getInstance(); // Object created!

  $objA-> sayHello(); // Hello my name is Freddy!

  $objA-> setName("Alex");

  $objA-> sayHello(); // Hello my name is Alex!

  $objB = MySingleton::getInstance();

  $objB-> sayHello(); // Hello my name is Alex!

  $objB-> setName("Bob");

  $objA-> sayHello(); // Hello my name is Bob!

?>
vvikramraj at yahoo dot com 23-Sep-2008 03:24
when attempting to implement a singleton class, one might also want to either
a) disable __clone by making it private
b) bash the user who attempts to clone by defining __clone to throw an exception
Mathijs Vos 23-Aug-2008 03:53
<?php
class foo
{
    public static $myStaticClass;
   
    public function __construct()
    {
        self::myStaticClass = new bar();
    }
}

class bar
{
        public function __construct(){}
}
?>

Please note, this won't work.
Use self::$myStaticClass = new bar(); instead of self::myStaticClass = new bar(); (note the $ sign).
Took me an hour to figure this out.
inkredibl 28-Jan-2008 12:27
Note that you should read "Variables/Variable scope" if you are looking for static keyword use for declaring static variables inside functions (or methods). I myself had this gap in my PHP knowledge until recently and had to google to find this out. I think this page should have a "See also" link to static function variables.
http://www.php.net/manual/en/language.variables.scope.php
ssj dot narutovash at gmail dot com 01-Jan-2008 08:48
It's come to my attention that you cannot use a static member in an HEREDOC string.  The following code

class A
{
  public static $BLAH = "user";

  function __construct()
  {
    echo <<<EOD
<h1>Hello {self::$BLAH}</h1>
EOD;
  }
}

$blah = new A();

produces this in the source code:

<h1>Hello {self::}</h1>

Solution:

before using a static member, store it in a local variable, like so:

class B
{
  public static $BLAH = "user";

  function __construct()
  {
    $blah = self::$BLAH;
    echo <<<EOD
<h1>Hello {$blah}</h1>
EOD;
  }
}

and the output's source code will be:

<h1>Hello user</h1>
aidan at php dot net 04-May-2005 07:14
To check if a function was called statically or not, you'll need to do:

<?php
function foo () {
    $isStatic = !(isset($this) && get_class($this) == __CLASS__);
}
?>

More at (http://blog.phpdoc.info/archives/4-Schizophrenic-Methods.html).

(I'll add this to the manual soon).
06-Apr-2005 03:14
You misunderstand the meaning of inheritance : there is no duplication of members when you inherit from a base class. Members are shared through inheritance, and can be accessed by derived classes according to visibility (public, protected, private).

The difference between static and non static members is only that a non static member is tied to an instance of a class although a static member is tied to the class, and not to a particular instance.
That is, a static member is shared by all instances of a class although a non static member exists for each instance of  class.

Thus, in your example, the static property has the correct value, according to principles of object oriented conception.
class Base
{
  public $a;
  public static $b;
}

class Derived extends Base
{
  public function __construct()
  {
    $this->a = 0;
    parent::$b = 0;
  }
  public function f()
  {
    $this->a++;
    parent::$b++;
  }
}

$i1 = new Derived;
$i2 = new Derived;

$i1->f();
echo $i1->a, ' ', Derived::$b, "\n";
$i2->f();
echo $i2->a, ' ', Derived::$b, "\n";

outputs
1 1
1 2
michalf at ncac dot torun dot pl 31-Mar-2005 02:42
Inheritance with the static elements is a nightmare in php. Consider the following code:

<?php
class BaseClass{
    public static $property;
}

class DerivedClassOne extends BaseClass{
}

class DerivedClassTwo extends BaseClass{
}

DerivedClassOne::$property = "foo";
DerivedClassTwo::$property = "bar";

echo DerivedClassOne::$property; //one would naively expect "foo"...
?>

What would you expect as an output? "foo"? wrong. It is "bar"!!! Static variables are not inherited, they point to the BaseClass::$property.

At this point I think it is a big pity inheritance does not work in case of static variables/methods. Keep this in mind and save your time when debugging.

best regards - michal
michael at digitalgnosis dot removethis dot com 15-Dec-2004 11:09
If you are trying to write classes that do this:

<?php

class Base
{
    static function Foo ()
    {
        self::Bar();
    }
}

class Derived extends Base
{
    function Bar ()
    {
        echo "Derived::Bar()";
    }
}

Derived::Foo(); // we want this to print "Derived::Bar()"

?>

Then you'll find that PHP can't (unless somebody knows the Right Way?) since 'self::' refers to the class which owns the /code/, not the actual class which is called at runtime. (__CLASS__ doesn't work either, because: A. it cannot appear before ::, and B. it behaves like 'self')

But if you must, then here's a (only slightly nasty) workaround:

<?php

class Base
{
    function Foo ( $class = __CLASS__ )
    {
        call_user_func(array($class,'Bar'));
    }
}

class Derived extends Base
{
    function Foo ( $class = __CLASS__ )
    {
        parent::Foo($class);
    }

    function Bar ()
    {
        echo "Derived::Bar()";
    }
}

Derived::Foo(); // This time it works. 

?>

Note that Base::Foo() may no longer be declared 'static' since static methods cannot be overridden (this means it will trigger errors if error level includes E_STRICT.)

If Foo() takes parameters then list them before $class=__CLASS__ and in most cases, you can just forget about that parameter throughout your code.

The major caveat is, of course, that you must override Foo() in every subclass and must always include the $class parameter when calling parent::Foo().