Constants
Constants allow a programmer to write a value that is going to remain constant in a way that cannot be changed.
This means unlike a variable a constant will always be equal to that value and cannot be assigned to.
Therefore you could have a constant called
SITENAME, by using the
define function
which allows constants to be defined.
define('SITENAME', 'PHP Rocks!');
echo SITENAME;
This will output
PHP Rocks!. If you write a variable name accidently without a $ sign PHP will assume that
you have tried to access a constant so you may get a "Undefined constant" error!
As a constant cannot be assigned to, you cannot assign a value to a constant twice. If you do so it will cause a error,
to get around this you can used the
defined function that will return true if a constant has already been
defined.
define('SITENAME', 'PHP Rocks!');
if (!defined('SITENAME'))
{
define('SITENAME', 'PHP Rocks a lot!');
}
echo SITENAME;
The above code won't error. Note that a constant can contain letters, numbers and underscores but not start with a number.
Sometimes you need to return the value of a constant when you only have its name as a string, to do this you use the
constant
function.
$name = "SITENAME";
echo constant($name);
This will return the value of the constant SITENAME.
Class and Interface constants
Classes and interfaces can also have constants and are visible only via the class or interface. To define a class or interface
constant do the following.
//for interfaces
interface People
{
const CODE = 'Person#';
}
//for classes
class Person
{
const SPACE = ' ';
}
These can be accessed from within a class like so.
//for classes
class Person
{
const SPACE = ' ';
private $firstname;
private $surname;
function name()
{
return $this->firstname . self::SPACE . $this->surname;
}
}
From outside the class or interface you can access it like so.
//for classes
echo Person::SPACE;
//for interfaces
echo People::CODE;
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Written by Dominic Skinner
Last Updated: 2011-10-25 16:00:38
