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
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
Use get_defined_constants() to get a list of
all defined constants.
Constants and (global) variables are in a different namespace.
This implies that for example
$TRUE are generally different.
If you use an undefined constant, PHP assumes that you mean
the name of the constant itself, just as if you called it as
a string (CONSTANT vs "CONSTANT"). An error of level
E_NOTICE will be issued
when this happens. See also the manual entry on why
wrong (unless you first define()
bar as a constant). If you simply want to check if a
constant is set, use the defined() function.
These are the differences between constants and variables:
Constants do not have a dollar sign ($)
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
Constants may only evaluate to scalar values.
Example #1 Defining Constants
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
// Works as of PHP 5.3.0
const CONSTANT = 'Hello World';
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
See also Class Constants.