类常量
可以把在类中始终保持不变的值定义为 常量 。
类常量的默认可见性是 public 。
注意:
类常量可以通过子类重新定义。PHP 8.1.0 起，如果类常量定义为 final，则不能被子类重新定义。
接口（interface）中也可以定义常量。更多示例见文档中的接口部分。
可以用一个变量来动态调用类。但该变量的值不能为关键字（如 self ，
parent 或 static）。
注意，类常量只为每个类分配一次，而不是为每个类的实例分配。
示例 #1 定义和使用一个类常量
<?php
class MyClass
{
const CONSTANT = 'constant value';
function showConstant() {
echo self::CONSTANT . "\n";
}
}
echo MyClass::CONSTANT . "\n";
$classname = "MyClass";
echo $classname::CONSTANT . "\n";
$class = new MyClass();
$class->showConstant();
echo $class::CONSTANT."\n";
?>
特殊的 ::class 允许在编译时进行完全限定的类名解析，
这在命名空间类中非常有用。
示例 #2 命名空间的 ::class 示例
<?php
namespace foo {
class bar {
}
echo bar::class; // foo\bar
}
?>示例 #3 类常量表达式示例
<?php
const ONE = 1;
class foo {
const TWO = ONE * 2;
const THREE = ONE + self::TWO;
const SENTENCE = 'The value of THREE is '.self::THREE;
}
?>示例 #4 自 PHP 7.1.0 起，类常量支持可见性修饰符
<?php
class Foo {
public const BAR = 'bar';
private const BAZ = 'baz';
}
echo Foo::BAR, PHP_EOL;
echo Foo::BAZ, PHP_EOL;
?>以上示例在 PHP 7.1 中的输出：
bar Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …
注意:
自 PHP 7.1.0 起，类常量允许使用可见性修饰符。
示例 #5 自 PHP 8.3.0 起，类常量可见性不一致检查
<?php
interface MyInterface
{
public const VALUE = 42;
}
class MyClass implements MyInterface
{
protected const VALUE = 42;
}
?>以上示例在 PHP 8.3 中的输出：
Fatal error: Access level to MyClass::VALUE must be public (as in interface MyInterface) …
注意: 自 PHP 8.3.0 起，可见性差异检查要更加严格。在此版本之前，类常量的可见性可能与实现的接口中常量的可见性不同。
示例 #6 自 PHP 8.3.0 起获取类常量语法
<?php
class Foo {
public const BAR = 'bar';
private const BAZ = 'baz';
}
$name = 'BAR';
echo Foo::{$name}, PHP_EOL; // bar
?>注意:
自 PHP 8.3.0 起，可以使用变量动态获取类常量。
＋添加备注
用户贡献的备注 12 notes
tmp dot 4 dot longoria at gmail dot com ¶
13 years ago
it's possible to declare constant in base class, and override it in child, and access to correct value of the const from the static method is possible by 'get_called_class' method:
<?php
abstract class dbObject
{
const TABLE_NAME='undefined';
public static function GetAll()
{
$c = get_called_class();
return "SELECT * FROM `".$c::TABLE_NAME."`";
}
}
class dbPerson extends dbObject
{
const TABLE_NAME='persons';
}
class dbAdmin extends dbPerson
{
const TABLE_NAME='admins';
}
echo dbPerson::GetAll()."<br>";//output: "SELECT * FROM `persons`"
echo dbAdmin::GetAll()."<br>";//output: "SELECT * FROM `admins`"
?>
anonymous ¶
14 years ago
Most people miss the point in declaring constants and confuse then things by trying to declare things like functions or arrays as constants. What happens next is to try things that are more complicated then necessary and sometimes lead to bad coding practices. Let me explain...
A constant is a name for a value (but it's NOT a variable), that usually will be replaced in the code while it gets COMPILED and NOT at runtime.
So returned values from functions can't be used, because they will return a value only at runtime.
Arrays can't be used, because they are data structures that exist at runtime.
One main purpose of declaring a constant is usually using a value in your code, that you can replace easily in one place without looking for all the occurences. Another is, to avoid mistakes.
Think about some examples written by some before me:
1. const MY_ARR = "return array(\"A\", \"B\", \"C\", \"D\");";
It was said, this would declare an array that can be used with eval. WRONG! This is just a string as constant, NOT an array. Does it make sense if it would be possible to declare an array as constant? Probably not. Instead declare the values of the array as constants and make an array variable.
2. const magic_quotes = (bool)get_magic_quotes_gpc();
This can't work, of course. And it doesn't make sense either. The function already returns the value, there is no purpose in declaring a constant for the same thing.
3. Someone spoke about "dynamic" assignments to constants. What? There are no dynamic assignments to constants, runtime assignments work _only_ with variables. Let's take the proposed example:
<?php
/**
* Constants that deal only with the database
*/
class DbConstant extends aClassConstant {
protected $host = 'localhost';
protected $user = 'user';
protected $password = 'pass';
protected $database = 'db';
protected $time;
function __construct() {
$this->time = time() + 1; // dynamic assignment
}
}
?>
Those aren't constants, those are properties of the class. Something like "this->time = time()" would even totally defy the purpose of a constant. Constants are supposed to be just that, constant values, on every execution. They are not supposed to change every time a script runs or a class is instantiated.
Conclusion: Don't try to reinvent constants as variables. If constants don't work, just use variables. Then you don't need to reinvent methods to achieve things for what is already there.
kuzawinski dot marcin at gmail dot com ¶
10 years ago
As of PHP 5.6 you can finally define constant using math expressions, like this one:
<?php
class MyTimer {
const SEC_PER_DAY = 60 * 60 * 24;
}
?>
Me happy :)
delete dot this dot and dot dots dot gt at kani dot hu ¶
11 years ago
I think it's useful if we draw some attention to late static binding here:
<?php
class A {
const MY_CONST = false;
public function my_const_self() {
return self::MY_CONST;
}
public function my_const_static() {
return static::MY_CONST;
}
}
class B extends A {
const MY_CONST = true;
}
$b = new B();
echo $b->my_const_self ? 'yes' : 'no'; // output: no
echo $b->my_const_static ? 'yes' : 'no'; // output: yes
?>
Xiong Chiamiov ¶
10 years ago
const can also be used directly in namespaces, a feature never explicitly stated in the documentation.
<?php
# foo.php
namespace Foo;
const BAR = 1;
?>
<?php
# bar.php
require 'foo.php';
var_dump(Foo\BAR); // => int(1)
?>
jimmmy dot chief at gmail dot com ¶
8 years ago
Hi, i would like to point out difference between self::CONST and $this::CONST with extended class.
Let us have class a:
<?php
class a {
const CONST_INT = 10;
public function getSelf(){
return self::CONST_INT;
}
public function getThis(){
return $this::CONST_INT;
}
}
?>
And class b (which extends a)
<?php
class b extends a {
const CONST_INT = 20;
public function getSelf(){
return parent::getSelf();
}
public function getThis(){
return parent::getThis();
}
}
?>
Both classes have same named constant CONST_INT.
When child call method in parent class, there is different output between self and $this usage.
<?php
$b = new b();
print_r($b->getSelf()); //10
print_r($b->getThis()); //20
?>
powtac at gmx dot de ¶
1 year ago
Since it it is not mentioned in the description the following types can be set as a class constant: string, array, int, bool and probably float. But not objects.
<?php
class Test {
const arr = array();
const string = 'string';
const int = 99;
const bool = true;
}
var_dump(
(new Test())::arr,
(new Test())::string,
(new Test())::int,
(new Test())::bool
);
/* ouput for PHP 7.0.0+:
array(0) {
}
string(6) "string"
int(99)
bool(true)
nepomuk at nepda dot de ¶
8 years ago
[Editor's note: that is already possible as of PHP 5.6.0.]
Note, as of PHP7 it is possible to define class constants with an array.
<?php
class MyClass
{
const ABC = array('A', 'B', 'C');
const A = '1';
const B = '2';
const C = '3';
const NUMBERS = array(
self::A,
self::B,
self::C,
);
}
var_dump(MyClass::ABC);
var_dump(MyClass::NUMBERS);
// Result:
/*
array(3) {
[0]=>
string(1) "A"
[1]=>
string(1) "B"
[2]=>
string(1) "C"
}
array(3) {
[0]=>
string(1) "1"
[1]=>
string(1) "2"
[2]=>
string(1) "3"
}
*/
?>
David Spector ¶
6 years ago
The usual comma-separated syntax can be used to declare several constants:
class STATE
{
const INIT=0, NAME_SEEN=1, ADDR_SEEN=2;
}
This shows the declaration of a set of enumeration literals suitable for use in a finite state machine loop. Reference such an enum by using syntax such as "STATE::INIT". Its actual type in this case will be integer.
Nimja ¶
7 years ago
Note that this magic constant DOES NOT load classes. And in fact can work on classes that do not exist.
This means it does not mess with auto-loading.
<?php
$className = \Foo\Bar::class;
var_dump($className);
var_dump(class_exists($className, false));
?>
Will output:
string(7) "Foo\Bar"
bool(false)
wbcarts at juno dot com ¶
16 years ago
Use CONST to set UPPER and LOWER LIMITS
If you have code that accepts user input or you just need to make sure input is acceptable, you can use constants to set upper and lower limits. Note: a static function that enforces your limits is highly recommended... sniff the clamp() function below for a taste.
<?php
class Dimension
{
const MIN = 0, MAX = 800;
public $width, $height;
public function __construct($w = 0, $h = 0){
$this->width = self::clamp($w);
$this->height = self::clamp($h);
}
public function __toString(){
return "Dimension [width=$this->width, height=$this->height]";
}
protected static function clamp($value){
if($value < self::MIN) $value = self::MIN;
if($value > self::MAX) $value = self::MAX;
return $value;
}
}
echo (new Dimension()) . '<br>';
echo (new Dimension(1500, 97)) . '<br>';
echo (new Dimension(14, -20)) . '<br>';
echo (new Dimension(240, 80)) . '<br>';
?>
- - - - - - - -
Dimension [width=0, height=0] - default size
Dimension [width=800, height=97] - width has been clamped to MAX
Dimension [width=14, height=0] - height has been clamped to MIN
Dimension [width=240, height=80] - width and height unchanged
- - - - - - - -
Setting upper and lower limits on your classes also help your objects make sense. For example, it is not possible for the width or height of a Dimension to be negative. It is up to you to keep phoney input from corrupting your objects, and to avoid potential errors and exceptions in other parts of your code.
AamirSohailKmAs at gmail dot com ¶
2 months ago
Note
as of PHP 8.3, it is possible to define Typed class constants.
RFC: https://wiki.php.net/rfc/typed_class_constants
<?php
class Example {
public const string NAME = "Aamir";
public const int MAX = 100;
}
?>备份地址：http://www.lvesu.com/blog/php/language.oop5.constants.php