What do two colons mean in PHP?
I don't know what it's doing when w开发者_运维问答e have this situation:
Foo::Bar
It looks like a path.
The ::
operator is the scope resolution operator. It is used to access class constants or static properties and methods, either from outside the class:
ClassName::CONSTANT_VALUE
ClassName::staticMethod()
Or within a class method to reference the same or a parent class using self
and parent
:
self::CONSTANT_VALUE
self::staticMethod()
parent::CONSTANT_VALUE
parent::staticMethod()
That's (generally) for accessing a static method or property in a class. It's called the scope resolution operator, or Paamayim Nekudotayim (which leads to some amazingly confusing error messages!). See http://www.php.net/manual/en/language.oop5.paamayim-nekudotayim.php.
The Scope Resolution Operator(::)the double colon, is a token that allows access to static, constant, and overridden properties or methods of a class.
<?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
?>
To supplement the answers regarding PHP's use of two colons as its "scope resolution operator":
In addition, a double colon is used:
To resolve an unqualified, qualified, or aliased class name to its fully qualified form, and
To invoke a class's "__callStatic" method with an arbitrary, previously undeclared method name.
To resolve a class name to its fully qualified form by appending "::class"
Two colons followed by the "class" keyword, placed after the name of a class, provides that class's fully qualified name as a string. I.e., "ClassName::class" resolves to the fully qualified name of "ClassName". See: (A) Manual: Classes and Objects: Basics, (B) Manual: Classes and Objects: Class Constants, and (C) Manual: Language Reference: Constants
The syntax was adopted in PHP 5.5. See: (A) RFC and (B) PHP 5.5 New Features
The "::class" syntax is useful within a namespace to obtain the fully qualified name of a class from its unqualified or qualified form, or from an alias of its name.
The "::class" syntax seems to work to resolve interface names as well as class names, although that does not appear to be documented by the sources linked above.
Within a class, the syntax also works with "self::class", as mentioned by the "::class" RFC linked above.
A few examples:
<?php
namespace MyNamespace;
use MyNamespace\YourInterface as HerInterface;
use MyNamespace\YourClass as HerClass;
use MyNamespace\TheirClass as OurClass;
interface MyInterface { }
interface YourInterface { }
class MyClass { }
class YourClass { }
class TheirClass
{
public function fullName()
{
echo self::class;
}
}
$ourClassInstance = new OurClass;
echo MyClass::class, PHP_EOL;
// outputs: MyNamespace\MyClass
echo HerClass::class, PHP_EOL;
// outputs: MyNamespace\YourClass
echo MyInterface::class, PHP_EOL;
// outputs: MyNamespace\MyInterface
echo HerInterface::class, PHP_EOL;
// outputs: MyNamespace\YourInterface
echo $ourClassInstance->fullName(), PHP_EOL;
// outputs: MyNamespace\TheirClass
To invoke "__callStatic" with an undeclared method name
Two colons can be used to "call" a static method name that a class has not declared. E.g., "ClassName::arbitraryMethodName()". Doing so invokes the class's "__callStatic" method, if the class has declared one. It also passes to __callStatic the name of the undeclared method and any arguments passed to the undeclared method. The __callStatic method then may "dynamically" choose how to handle the call. PHP refers to this as "overloading" with the __callStatic "magic method".
See additional StackOverflow discussion
Example:
<?php
namespace OurCompany\Orders;
class Intake
{
public static function __callStatic($name, $arguments)
{
$item = substr($name, 5); // trims "order" prefix
$specialistClass = "\OurCompany\Specialists\\" . $item;
if (class_exists($specialistClass)) {
$specialist = new $specialistClass;
return $specialist->handleOrder($arguments);
}
return "I'm sorry, we can't help you with " .
lcfirst($item) . ".";
}
}
namespace OurCompany\Specialists;
class Car
{
public function handleOrder($arguments)
{
return "May I help you with a $arguments[0] car?";
}
}
class Truck
{
public function handleOrder($arguments)
{
return "May I help you with a $arguments[0] truck?";
}
}
use OurCompany\Orders\Intake;
echo Intake::orderCar("red"), PHP_EOL;
// outputs: May I help you with a red car?
echo Intake::orderTruck("pickup"), PHP_EOL;
// outputs: May I help you with a pickup truck?
echo Intake::orderShoes("suede"), PHP_EOL;
// outputs: I'm sorry, we can't help you with shoes.
use of Scope Resolution Operator
A class constant, class property (static), class function (static) can all share the same name and be accessed using the double-colon
class B{
public static $par = "par";
}
class A extends B{
const B = "constant";
public static $sel = "self";
public static $b = "static property";
public static function b(){
echo "static function";
}
public static function c(){
return parent::$par;
}
public static function d(){
return self::$sel;
}
public static function e(){
return self::$par;
}
}
echo A::B.PHP_EOL;
echo A::$b.PHP_EOL;
echo A::b().PHP_EOL;
echo A::c().PHP_EOL;
echo A::d().PHP_EOL;
精彩评论