Constants | Variables And Data Types

Introduction to Constants

In PHP, constants are defined values that remain unchanged throughout the execution of a script. Unlike variables, constants cannot be altered once they are set. They are useful for defining unchangeable values such as configuration settings, fixed strings, or numeric values that should remain constant.

Defining Constants

Constants are defined using the define() function. The syntax is:

define("CONSTANT_NAME", "value");

Here, "CONSTANT_NAME" is the name of the constant, and "value" is the value assigned to the constant.

Example of Defining and Using Constants

Let's look at an example where we define a constant and use it in a script:


                    <?php

                    define("SITE_NAME", "My Awesome Website");

                    echo "Welcome to " . SITE_NAME;

                    ?>

Output:
Welcome to My Awesome Website

Rules for Naming Constants

When naming constants, follow these rules:

Constants should be named using uppercase letters to distinguish them from variables.
Constants do not need a dollar sign ($) before their name.
Constants can contain letters, digits, and underscores (_), but cannot start with a digit.

Magic Constants

PHP provides several predefined constants known as magic constants. These constants change based on where they are used. Some of the commonly used magic constants are:

__LINE__ - The current line number of the file.
__FILE__ - The full path and filename of the file.
__DIR__ - The directory of the file.
__FUNCTION__ - The function name.
__CLASS__ - The class name.
__METHOD__ - The class method name.
__NAMESPACE__ - The name of the current namespace.

Example of Using Magic Constants

Here is an example demonstrating the use of some magic constants:


                    <?php

                    echo "This is line number " . __LINE__ . "<br>";

                    echo "The full path of this file is " . __FILE__ . "<br>";

                    echo "The directory of this file is " . __DIR__ . "<br>";

                    ?>

Output:
This is line number 2
The full path of this file is /path/to/your/file.php
The directory of this file is /path/to/your

Conclusion

Constants in PHP are a powerful tool for defining immutable values that are used throughout your code. They help make your code more readable and maintainable by providing a single point of reference for critical values that should not change. By understanding and using constants effectively, you can write more robust and reliable PHP applications.

PHP Development: Constants