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:
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.
Instead of using the echo statement ,there is an another way to print constants using the constant() function.
Constants are Global: By default, constants are automatically global, and can be used throughout the script, accessible inside and outside of any function.
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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