PHP Constants

In this tutorial, you will learn how to use constants to store fixed values in PHP.

What are constants in PHP

Constants are names or identifiers of fixed values. Constants are similar to variables, except that once a constant is defined, it cannot be undefined or changed (Magic constantsexcept).

Constants are very useful for storing data that will not change during the execution of the script. Common examples of such data include configuration settings, such as database usernames and passwords, the basic URL of the website, company name, etc.

Constants are defined using PHP's define() function, which accepts two parameters: the name of the constant and its value. Once the constant value is defined, it can be accessed at any time by referencing its name. Here is a simple example:

<?php
//　Define constants
define("SITE_URL",　"https://www.oldtoolbag.com/");
　
//　Using constants
echo　'Thank you for visiting　-　'　.　SITE_URL;
?>

The output of the above code will be:

Thank you for visiting-https://www.oldtoolbag.com/

The PHP echo statement is usually used to display data or output data to the web browser. We will learn about this statement in detail in the next chapter.

Tip:By storing values in constants instead of variables, it can ensure that the value will not be unexpectedly changed during the execution of the application.

The difference between constants and variables is

• A dollar sign ($) does not need to be written before constants, but it must be written before the name of a variable.

• Constants cannot be defined by simple assignment; they must be defined using the define() function.

• Constants can be defined and accessed at any location without considering variable scope rules.

• Once a constant is set, it cannot be redefined or undefined.

Valid and invalid constant names

//　Valid constant names
define("ONE",              "first thing");
define("TWO2",          "second thing");
define("THREE_3",          "third thing");
define("__THREE__",  "third value");
//　Invalid constant name
define("2TWO",          "second thing");

PHP Magic Constants

PHP provides a large number of predefined constants for any script it runs.
There are five magic constants that change according to the position used. For example, the value of __LINE__ depends on the line used in the script. These special constants are not case-sensitive, as shown below-
Below are some 'magic' PHP constants-

PHP naming conventions for constants

Constant names must follow the sameVariable namesThe same rule applies, which means that valid constant names must start with a letter or underscore, followed by any number of letters, numbers, or underscores, but there is an exception:Constant names do not require a prefix of $