PHP 7.1.0 Released

Constantes do Objeto

É possível definir valores constantes em cada classe permanecendo a mesma e imutável. Constantes diferem de variáveis normais, ao não usar o símbolo $ para declará-las ou usá-las. A visibilidade padrão de constantes de classe é public.

O valor deve ser uma expressão constante, não podendo ser (por exemplo) uma variável, uma propriedade, ou uma chamada a uma função.

Também é possível que interfaces tenham constantes. Veja na documentação de interface os exemplos.

A partir do PHP 5.3.0, é possível referenciar a classe usando uma variável. O valor da variável não pode ser uma palavra-chave (e.g. self, parent e static).

Constantes de classe são alocadas por classe, e não em cada instância da classe.

Exemplo #1 Definindo e usando uma constante

<?php
class MinhaClasse
{
    const 
constante 'valor constante';

    function 
mostrarConstante() {
        echo  
self::constante "\n";
    }
}

echo 
MinhaClasse::constante "\n";

$classname "MinhaClasse";
echo 
$classname::constante// A partir do PHP 5.3.0

$classe = new MinhaClasse();
$classe->mostrarConstante();

echo 
$classe::constante// A partir do PHP 5.3.0
?>

Exemplo #2 Exemplo com informação estática

<?php
class foo {
    
// A partir do PHP 5.3.0
    
const BAR = <<<'EOT'
bar
EOT;
    
// A partir do 5.3.0
    
const BAZ = <<<EOT
baz
EOT;
}
?>

Nota:

O suporte para inicialização de constantes com Heredoc e Nowdoc foi adicionado no PHP 5.3.0.

Exemplo #3 Exemplo de expressão constante

<?php
const ONE 1;

class 
foo {
    
// A partir do 5.6.0
    
const TWO ONE 2;
    const 
THREE ONE self::TWO;
    const 
SENTENCE 'O valor de THREE é '.self::THREE;
}
?>

É possível fornecer uma expressão escalar envolvendo strings e numéricos literais e/ou constantes no contexto de uma constante de classe.

Nota:

Expressões constantes foram adicionados no PHP 5.6.0.

Exemplo #4 Modificadores de visibilidade em contantes de classe

<?php
class Foo {
    
// As of PHP 7.1.0
    
public const BAR 'bar';
    private const 
BAZ 'baz';
}
echo 
Foo::BARPHP_EOL;
echo 
Foo::BAZPHP_EOL;
?>

O exemplo acima irá imprimir no PHP 7.1:

bar

Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …

Nota:

Modificadores de visibilidade estão disponíveis desde o PHP 7.1.0.

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User Contributed Notes 15 notes

up
78
kuzawinski dot marcin at gmail dot com
2 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 :)
up
95
tmp dot 4 dot longoria at gmail dot com
5 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`"

?>
up
58
Xiong Chiamiov
2 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)
?>
up
68
anonymous
6 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.
up
58
delete dot this dot and dot dots dot gt at kani dot hu
2 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
?>
up
9
nepomuk at nepda dot de
8 months 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"
}
*/
?>
up
14
wbcarts at juno dot com
8 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.
up
9
keenskelly at gmail dot com
2 years ago
Re: "The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call."

I dare say that "a mathematical operation" can indeed be a constant expression. I was quite surprised by this limitation; you cannot, for example do something like:

const LIMITMB = 20;
const LIMITB = LIMITMB * 1024 * 1024;

It is very common to be able to express something like that in other languages, like C with #defines, where changing one definition has a cascading effect on others without having to pre-calculate hard-coded numbers all over the place. So beware, you might be better off using a private static or global contstant definition if you need to do anything more sophisticated than a name=primitive value pair.
up
3
Paul
1 year ago
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.
up
1
enrico_kaelert at kabelmail dot com
2 years ago
additional to tmp dot 4 dot longoria at gmail dot com ´s post:
quote:
it's possible to declare constant in base class, and override it in child,
/quote

Its not that we overwrite them.
Its more that each got its own:
<?php
abstract class dbObject
{
    const
TABLE_NAME='undefined';
}

class
dbPerson extends dbObject
{
    const
TABLE_NAME='persons';

    public static function
getSelf()
    {
        return
self::TABLE_NAME;
    }
    public static function
getParent()
    {
        return
parent::TABLE_NAME;
    }
}

class
dbAdmin extends dbPerson
{
    const
TABLE_NAME='admins';

    public static function
getSelf()
    {
        return
self::TABLE_NAME;
    }
    public static function
getParent()
    {
        return
parent::TABLE_NAME;
    }
}

echo
'<pre>
im class dbPerson{} and this is my:
    self TABLE_NAME:    '
.dbPerson::getSelf().'   // persons
    parent TABLE_NAME: '
.dbPerson::getParent().'  // undefined

im class dbAdmin{} and this is my:
    self TABLE_NAME:   '
.dbAdmin::getSelf().'    // admins
    parent TABLE_NAME: '
.dbAdmin::getParent().'  // persons
'
;
?>

or more readable:
<?php
class ParentClass
{
    const
CONSTANT = 'CONST_PARENT';
}

class
A extends ParentClass
{
    const
CONSTANT = 'CONST_A';

    public static function
getSelf()
    {
        return
self::CONSTANT;
    }
    public static function
getParent()
    {
        return
parent::CONSTANT;
    }
}

echo
'<pre>
im class A{} and this is my:
    self CONSTANT:    '
.A::getSelf().'   // CONST_A
    parent CONSTANT: '
.A::getParent().'  // CONST_PARENT
'
;
?>
up
3
Anonymous
3 years ago
Noted by another is that class constants take up memory for every instance. I cannot see this functionality being accurate, so testing thusly:

class SomeClass {
const thing = 0;
const thing2 = 1;
}

$m0 = memory_get_usage();
$p0 = new SomeClass();
$p1 = new SomeClass();
$p2 = new SomeClass();
$m1 = memory_get_usage();
printf("memory %d<br />", $m1 - $m0);

The output does not change when one alters the count of constants in "SomeClass".
up
1
jimmmy dot chief at gmail dot com
1 month 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

?>
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-7
jaimz at vertigolabs dot org
3 years ago
I thought it would be relevant to point out that with php 5.5, you can not use self::class, static::class, or parent::class to produce a FQN. Doing so produces a PHP Parse error:

"PHP Parse error:  syntax error, unexpected 'class' (T_CLASS), expecting variable (T_VARIABLE) or '$'"

It would be nice if you could do this however.
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-10
jakub dot lopuszanski at nasza-klasa dot pl
6 years ago
[Editor's note: that behavior has changed as of PHP 7.0.0, though.]

Suprisingly consts are lazy bound even though you use self instead of static:
<?php
class A{
  const
X=1;
  const
Y=self::X;
}
class
B extends A{
  const
X=1.0;
}
var_dump(B::Y); // float(1.0)
?>
up
-4
info at stanzentech dot com
1 year ago
<?php
//http://php.net/manual/en/language.oop5.constants.php
/**
* Constant name shouldn't start with $
* Constant name may lower or uppercases.
* Same constant name can be used as a property name but must start with $ symbol.
* Constant doesn't available with $this-> inside class definition.
* Constant is available with self:: inside class definition.
* Constant can't call with $this-> outside class.
* Constant is accessible with :: after "Class Name or Object".
*
*/
class MyClass
{       
   
// Parse error: syntax error, unexpected '$CONSTANT' (T_VARIABLE), expecting identifier (T_STRING) in constant.php
    //const $CONSTANT      =   'constant named "CONSTANT" ';
 
   
const CONSTANT      =   'constant named "CONSTANT" ';
    const
small         =   'constant named "small" ';
   
    public
$small        =   'SAME CONTSNAT NAME AS PROPERTIES.'

   
//Fatal error: Cannot redefine class constant MyClass::small in constant.php
    // const small         =   'constant named "small" ';
       
   
function showConstant() {
        echo 
self::CONSTANT . "<br>";
       
//echo $this->CONSTANT . "<br>"; // Notice: Undefined property: MyClass::$CONSTANT in constant.php
   
}
}

$class      =   new MyClass();
$class->showConstant();

//Notice: Undefined property: MyClass::$CONSTANT in constant.php
//echo $class->CONSTANT."<br>";

echo $class->small."<br>"// SAME CONTSNAT NAME AS PROPERTIES.
?>
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