Константы в объектах

Константы также могут быть объявлены и в пределах одного класса. Отличие переменных и констант состоит в том, что при объявлении последних или при обращении к ним не используется символ $. Как и Ключевое слово "static" свойства и методы, значения констант, объявленных внутри класса, не могут быть получены через переменную, содержащую экземпляр этого класса.

Пример #1 Объявление и использование константы

<?php
class MyClass {
  const 
constant 'значение константы';

  function 
showConstant() {
    echo  
self::constant "\n";
  }
}

echo 
MyClass::constant "\n";

$class = new MyClass();
$class->showConstant();
/* echo $class::constant; -  такое использование недопустимо */
?>

Коментарии

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 0MAX 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(150097)) . '<br>';
echo (new 
Dimension(14, -20)) . '<br>';
echo (new 
Dimension(24080)) . '<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.
2008-09-12 15:12:09
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
Автор:
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.
2010-09-27 21:32:19
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
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`"

?>
2011-06-04 04:52:19
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
I think it's useful if we draw some attention to late static binding here:
<?php
class {
    const 
MY_CONST false;
    public function 
my_const_self() {
        return 
self::MY_CONST;
    } 
    public function 
my_const_static() {
        return static::
MY_CONST;
    } 
}

class 
extends {
   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
?>
2014-01-16 13:05:52
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
Автор:
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)
?>
2014-03-11 01:36:47
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
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 :)
2014-11-11 03:27:45
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
Автор:
Square or curly bracket syntax can normally be used to access a single byte (character) within a string. For example: $mystring[5]. However, please note that (for some reason) this syntax is not accepted for string class constants (at least, not in PHP 5.5.12).
For example, the following code gives "PHP Parse error:  syntax error, unexpected '[' in php shell code on line 6".
<?php
class SomeClass
{
  const 
SOME_STRING '0123456790';
  public static function 
ATest()
  {
    return 
self::SOME_STRING[0];
  }
}
?>
It looks like you have to use a variable/class member instead.
2015-05-12 13:44:57
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
[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 
'1';
    const 
'2';
    const 
'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"
}
*/
?>
2016-03-15 10:50:57
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
Hi, i would like to point out difference between self::CONST and $this::CONST with extended class.
Let us have class a:

<?php 
class {   
    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 extends {
    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

?>
2016-10-25 17:57:38
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
Автор:
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($classNamefalse));
?>

Will output: 

    string(7) "Foo\Bar"
    bool(false)
2017-09-06 16:07:31
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
Автор:
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.
2018-08-16 16:55:31
http://php5.kiev.ua/manual/ru/language.oop5.constants.html
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)
2023-05-05 17:02:08
http://php5.kiev.ua/manual/ru/language.oop5.constants.html

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