Constants are either identifiers or simple names that can be assigned any fixed values. They are similar to a variable except that they can never be changed. They remain constant throughout the program and cannot be altered during execution. Once a constant is defined, it cannot be undefined or redefined. Constant identifiers should be written in upper case following the convention. By default, a constant is always case-sensitive, unless mentioned. A constant name must never start with a number. It always starts with a letter or underscores, followed by letter, numbers or underscore. It should not contain any special characters except underscore, as mentioned.
Creating a PHP Constant
The define() function in PHP is used to create a constant as shown below:
Syntax:
define(name, value, case_insensitive)
The parameters are as follows:
- name: The name of the constant.
- value: The value to be stored in the constant.
- case_insensitive: Defines whether a constant is case insensitive. By default this value is False, i.e., case sensitive.
Example:
<?php // This creates a case-sensitive constant define("WELCOME", "neveropen"); echo WELCOME, "\n"; // This creates a case-insensitive constant define("HELLO", "neveropen", true); echo hello; ?> |
Output:
neveropen neveropen
constant() function
Instead of using the echo statement ,there is an another way to print constants using the constant() function.
Syntax
constant(name)
Example:
<?php define("WELCOME", "neveropen!!!"); echo WELCOME, "\n"; echo constant("WELCOME"); // same as previous ?> |
Output:
neveropen!!! neveropen!!!
Constants are Global: By default, constants are automatically global, and can be used throughout the script, accessible inside and outside of any function.
Example:
<?php define("WELCOME", "neveropen"); function testGlobal() { echo WELCOME; } testGlobal(); ?> |
neveropen
Constants vs Variables
- A constant, once defined can never be undefined but a variable can be easily undefined.
- There is no need to use dollar sign($) before constants during assignment but while declaring variables we use a dollar sign.
- A constant can only be defined using a define() function and not by any simple assignment.
- Constants dont need to follow any variable scoping rules and can be defined anywhere.
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