A constant is a identifier (name) for a simple value. As the name
suggests, that value cannot change during the execution of the
script. (The 'magic constants' __FILE__ and
__LINE__ appear to be an exception to this,
but they're not actually constants.) 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 thus:
[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*
Note:
For our purposes here, a letter is a-z, A-Z, and the ASCII
characters from 127 through 255 (0x7f-0xff).
The scope of a constant is global--you can access it anywhere in
your script without regard to 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,
double and string) can be contained
in constants.
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 are 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
$TRUE are generally different.
If you use an undefined constant, PHP assumes that you mean
the name of the constant itself. A
notice will be issued
when this happens. Use the defined()-function if
you want to know if a constant is set.
These are the differences between constants and variables:
Constants do not have a dollar sign ($)
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 8-1. Defining Constants <?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.
?> |
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