A constant is an identifier (name) for a simple
value. A constant value cannot change during the
execution of the script (except for magic constants, which aren't
A constant is case-sensitive by default. By
convention, constant identifiers are always uppercase.
The name of a constant follows the same rules as any label in PHP.
A valid constant name starts with a letter or underscore, followed
by any number of letters, numbers, or underscores. As a regular expression,
it would be expressed thusly: [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*
Valid and invalid constant names
// Valid constant names
define("FOO2", "something else");
define("FOO_BAR", "something more")
// Invalid constant names
// This is valid, but should be avoided:
// PHP may one day provide a magical constant
// that will break your script
Note: For our purposes here, a letter is a-z, A-Z, and the ASCII
characters from 127 through 255 (0x7f-0xff).
Like superglobals, the scope of a constant is global. You can access
constants anywhere in your script without regard to scope. For more
information on scope, read the manual section on variable scope.
You can define a constant by using the define()-function. Once
a constant is defined, it can never be changed or undefined.
Only scalar data (boolean, integer, float and string) can be contained
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 TRUE and $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 $foo[bar]
is wrong (unless you first define() bar as a constant). If you simply
want to check if a constant is set, use the defined() function.
The differences between constants and variables:
- Constants do not have a dollar sign ($) 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
- Constants may only evaluate to scalar values.
Example Defining Constants
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.