Syntax
You can define a constant by using the define()-function or by using the const keyword outside a class definition as of PHP 5.3.0. Once a constant is defined, it can never be changed or undefined.
Only scalar data (boolean, integer, float and string) can be contained in constants. It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results.
You can get the value of a constant by simply specifying its name. Unlike with variables, you should not prepend a constant with a $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name dynamically. Use get_defined_constants() to get a list of all defined constants.
Note: Constants and (global) variables are in a different namespace. This implies that for example
TRUEand $TRUE are generally different.
If you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a string (CONSTANT vs "CONSTANT"). An error of level E_NOTICE will be issued when this happens. See also the manual entry on why $foo[bar] is wrong (unless you first define() bar as a constant). If you simply want to check if a constant is set, use the defined() function.
These are the differences between constants and variables:
- Constants do not have a dollar sign ($) before them;
- Constants may only be defined using the define() function, not by simple assignment;
- Constants may be defined and accessed anywhere without regard to variable scoping rules;
- Constants may not be redefined or undefined once they have been set; and
- Constants may only evaluate to scalar values.
Example #1 Defining Constants
<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.
?>
Example #2 Defining Constants using the const keyword
<?php
// Works as of PHP 5.3.0
const CONSTANT = 'Hello World';
echo CONSTANT;
?>
Note:
As opposed to defining constants using define(), constants defined using the const keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops or if statements.
See also Class Constants.
Коментарии
Just a quick note:
From PHP7 on you can even define a multidimensional Array as Constant:
define('QUARTLIST',array('1. Quarter'=>array('jan','feb','mar'),'2.Quarter'=>array('may','jun','jul'));
does work as expected.
the documentation doesn't go too far in explaining the crucial difference between the two ways of declaring constants in PHP.
Const is handled at compile time, define() at run time. For this reason, a constant cannot be conditionally defined using Const, for example.
Another difference we can notice occurs in the constant declarations in classes. Const infiltrates the class scope, while define() leaks into the global scope.
<?php
Class Myclass (){
const NAME = "Nicolas";
}
?>
The NAME constant is within the scope of the MyClass class.