It may seem obvious, but class constants are always publicly visible. They cannot be made private or protected. I do not see it state that in the docs anywhere.
It is possible to define constant values on a per-class basis remaining the same and unchangeable. Constants differ from normal variables in that you don't use the $ symbol to declare or use them.
The value must be a constant expression, not (for example) a variable, a property, or a function call.
It's also possible for interfaces to have constants. Look at the interface documentation for examples.
As of PHP 5.3.0, it's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static).
Example #1 Defining and using a constant
<?php
class MyClass
{
const CONSTANT = 'constant value';
function showConstant() {
echo self::CONSTANT . "\n";
}
}
echo MyClass::CONSTANT . "\n";
$classname = "MyClass";
echo $classname::CONSTANT . "\n"; // As of PHP 5.3.0
$class = new MyClass();
$class->showConstant();
echo $class::CONSTANT."\n"; // As of PHP 5.3.0
?>
Example #2 Static data example
<?php
class foo {
// As of PHP 5.3.0
const BAR = <<<'EOT'
bar
EOT;
// As of PHP 5.3.0
const BAZ = <<<EOT
baz
EOT;
}
?>
Note:
Support for initializing constants with Heredoc and Nowdoc syntax was added in PHP 5.3.0.
Example #3 Constant expression example
<?php
const ONE = 1;
class foo {
// As of PHP 5.6.0
const TWO = ONE * 2;
const THREE = ONE + self::TWO;
const SENTENCE = 'The value of THREE is '.self::THREE;
}
?>
It is possible to provide a scalar expression involving numeric and string literals and/or constants in context of a class constant.
Note:
Constant expression support was added in PHP 5.6.0.
It may seem obvious, but class constants are always publicly visible. They cannot be made private or protected. I do not see it state that in the docs anywhere.
[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"
}
*/
?>
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 :)
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)
?>
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`"
?>
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
?>
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.
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.
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.
Class constants are allocated per instance of the class. If you create a class with 100 constants, each with 100 bytes, and 100 instances of that class, you will use 1 million bytes. Obviously that is a fringe case but remember that when you are creating constants that you might not need in every instance.
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.
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 - $m0);
The output does not change when one alters the count of constants in "SomeClass".
additional to tmp dot 4 dot longoria at gmail dot com ´s post:
quote:
it's possible to declare constant in base class, and override it in child,
/quote
Its not that we overwrite them.
Its more that each got its own:
<?php
abstract class dbObject
{
const TABLE_NAME='undefined';
}
class dbPerson extends dbObject
{
const TABLE_NAME='persons';
public static function getSelf()
{
return self::TABLE_NAME;
}
public static function getParent()
{
return parent::TABLE_NAME;
}
}
class dbAdmin extends dbPerson
{
const TABLE_NAME='admins';
public static function getSelf()
{
return self::TABLE_NAME;
}
public static function getParent()
{
return parent::TABLE_NAME;
}
}
echo '<pre>
im class dbPerson{} and this is my:
self TABLE_NAME: '.dbPerson::getSelf().' // persons
parent TABLE_NAME: '.dbPerson::getParent().' // undefined
im class dbAdmin{} and this is my:
self TABLE_NAME: '.dbAdmin::getSelf().' // admins
parent TABLE_NAME: '.dbAdmin::getParent().' // persons
';
?>
or more readable:
<?php
class ParentClass
{
const CONSTANT = 'CONST_PARENT';
}
class A extends ParentClass
{
const CONSTANT = 'CONST_A';
public static function getSelf()
{
return self::CONSTANT;
}
public static function getParent()
{
return parent::CONSTANT;
}
}
echo '<pre>
im class A{} and this is my:
self CONSTANT: '.A::getSelf().' // CONST_A
parent CONSTANT: '.A::getParent().' // CONST_PARENT
';
?>
The major problem of constants is for me, you cant use them for binary flags.
<?php
class constant {
const MODE_FLAG_1 = 1;
const MODE_FLAG_2 = 2;
const MODE_FLAG_3 = 4;
const DEFAULT_MODE = self::FLAG_1 | self::FLAG_2
private function foo ($mode=self::DEFAULT_MODE) {
// some operations
}
}
?>
This code will not work because constants can't be an calculation result. You could use
<?php
const DEFAULT_MODE = 3;
?>
instead, but we use flags to be value indipendent. So you would miss target with it. Only way is to use defines like ever before.
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 '$'"
It would be nice if you could do this however.
Suprisingly consts are lazy bound even though you use self instead of static:
<?php
class A{
const X=1;
const Y=self::X;
}
class B extends A{
const X=1.0;
}
var_dump(B::Y); // float(1.0)
?>
If you have a class which defines a constant which may be overridden in child definitions, here are two methods how the parent can access that constant:
<?php
class Weather
{
const danger = 'parent';
static function getDanger($class)
{
// Code to return the danger field from the given class name
}
}
class Rain extends Weather
{
const danger = 'child';
}
?>
The two options to place in the parent accessor are:
eval('$danger = ' . $class . '::danger;');
or:
$danger = constant($class . '::danger');
I prefer the last option, but they both seem to work.
So, why might this be useful? Well, in my case I have a page class which contains various common functions for all pages and specific page classes extend this parent class. The parent class has a static method which takes an argument (class name) and returns a new instantiation of the class.
Each child class has a constant which defines the access level the user must have in order to view the page. The parent must check this variable before creating and returning an instance of the child - the problem is that the class name is a variable and $class::danger will treat $class as an object.
A useful technique I've found is to use interfaces for package- or application-wide constants, making it easy to incorporate them into any classes that need access to them:
<?php
interface AppConstants
{
const FOOBAR = 'Hello, World.';
}
class Example implements AppConstants
{
public function test()
{
echo self::FOOBAR;
}
}
$obj = new Example();
$obj->test(); // outputs "Hello, world."
?>
I realize the same could be done simply by defining the constant in a class and accessing it via "class_name::const_name", but I find this a little nicer in that the class declaration makes it immediately obvious that you accessing values from the implemented interface.
class Country{
const IND = "India';
const PK='Pakistan';
contst NK='Nanhe kumar';
}
$countryCode='IND';
constant("Country::$countryCode");
$countryCode='PK';
constant("Country::$countryCode");
pre 5.3 can refer a class using variable and get constants with:
<?php
function get_class_const($class, $const){
return constant(sprintf('%s::%s', $class, $const));
}
class Foo{
const BAR = 'foobar';
}
$class = 'Foo';
echo get_class_const($class, 'BAR');
//'foobar'
?>
Since constants of a child class are not accessible from the parent class via self::CONST and there is no special keyword to access the constant (like this::CONST), i use private static variables and these two methods to make them read-only accessible from object's parent/child classes as well as statically from outside:
<?php
class b extends a {
private static $CONST = 'any value';
public static function getConstFromOutside($const) {
return self::$$const;
}
protected function getConst($const) {
return self::$$const;
}
}
?>
With those methods in the child class, you are now able to read the variables from the parent or child class:
<?php
class a {
private function readConst() {
return $this->getConst('CONST');
}
abstract public static function getConstFromOutside($const);
abstract protected function getConst($const);
}
?>
From outside of the object:
<?php
echo b::getConstFromOutside('CONST');
?>
You maybe want to put the methods into an interface.
However, class b's attribute $CONST is not a constant, so it is changeable by methods inside of class b, but it works for me and in my opinion, it is better than using real constants and accessing them by calling with eval:
<?php
protected function getConst($const) {
eval('$value = '.get_class($this).'::'.$const.';');
return $value;
}
?>
Lest anyone think this is somehow an omission in PHP, there is simply no point to having a protected or private constant. Access specifiers identify who has the right to *change* members, not who has the right to read them:
<?php
// define a test class
class Test
{
public static $open=2;
protected static $var=1;
private static $secret=3;
}
$classname="Test";
// reflect class information
$x=new ReflectionClass($classname);
$y=array();
foreach($x->GetStaticProperties() as $k=>$v)
$y[str_replace(chr(0),"@",$k)]=$v;
// define the variables to search for
$a=array("open","var","secret","nothing");
foreach($a as $b)
{
if(isset($y["$b"]))
echo "\"$b\" is public: {$y["$b"]}<br/>";
elseif(isset($y["@*@$b"]))
echo "\"$b\" is protected: {$y["@*@$b"]}<br/>";
elseif(isset($y["@$classname@$b"]))
echo "\"$b\" is private: {$y["@$classname@$b"]}<br/>";
else
echo "\"$b\" is not a static member of $classname<br/>";
}
?>
As you can see from the results of this code, the protected and private static members of Test are still visible if you know where to look. The protection and privacy are applicable only on writing, not reading -- and since nobody can write to a constant at all, assigning an access specifier to it is just redundant.
Note that since constants are tied to the class definition, they are static by definition and cannot be accessed using the -> operator.
A side effect of this is that it's entirely possible for a class constant to have the same name as a property (static or object):
<?php
class Foo
{
const foo = 'bar';
public $foo = 'foobar';
const bar = 'foo';
static $bar = 'foobar';
}
var_dump(foo::$bar); // static property
var_dump(foo::bar); // class constant
$bar = new Foo();
var_dump($bar->foo); // object property
var_dump(bar::foo); // class constant
?>
<?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.
?>
Using "const" in the global context works:
<?php
const FOO = 'bar';
var_dump(FOO);
?>