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Introduction to PHP Constants

Constants in PHP are identifiers for simple values that cannot be changed during the execution of a script. Unlike variables, constants are automatically global across the entire script.

Defining Constants

Constants are defined using the define() function or the const keyword.

Using define() function:


<?php
define("GREETING", "Welcome to PHP!");
echo GREETING;
?>

Using const keyword (PHP 5.3.0 and later):


<?php
const GREETING = "Welcome to PHP!";
echo GREETING;
?>

Note: The const keyword can be used to define constants at the top level of a script or inside a class definition, but not inside functions or loops.

Rules for PHP Constants

Constant names are case-sensitive by default. However, it's common practice to name constants in all uppercase letters.
Constants should start with a letter or underscore, followed by any number of letters, numbers, or underscores.
Once a constant is defined, it can't be changed or undefined.
Constants can only hold scalar values (boolean, integer, float and string) or arrays (PHP 7+).

Constants vs Variables

Here are the key differences between constants and variables:

Constants do not use the $ symbol before the identifier.
Constants can be defined and accessed anywhere in the script without regard to variable scoping rules.
Constants cannot be redefined or undefined once they have been set.
Constant values can only be strings and numbers (and arrays in PHP 7+).

Checking if a Constant is Defined

You can use the defined() function to check if a constant has been defined:


<?php
if (defined("GREETING")) {
    echo GREETING;
} else {
    echo "GREETING is not defined.";
}
?>

Magic Constants

PHP provides several predefined constants that change depending on where they are used. These are called magic constants:

__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 (PHP 4.3.0 and higher).
__CLASS__ - The class name (PHP 4.3.0 and higher).
__METHOD__ - The class method name (PHP 5.0.0 and higher).
__NAMESPACE__ - The name of the current namespace (PHP 5.3.0 and higher).

Example:


<?php
echo "Line number " . __LINE__ . " in file " . __FILE__;
?>

Constant Arrays (PHP 7+)

From PHP 7.0, you can create an array constant using the define() function:


<?php
define("ANIMALS", [
    "dog",
    "cat",
    "bird"
]);
echo ANIMALS[1]; // outputs "cat"
?>

Best Practices

Use constants for values that don't change throughout your script, like configuration settings.
Name constants in all uppercase to distinguish them from variables.
Group related constants together, possibly in a separate configuration file.
Use constants instead of "magic numbers" or string literals in your code to improve readability and maintainability.

Conclusion

Constants are an essential part of PHP programming, providing a way to store values that remain unchanged throughout script execution. They enhance code readability, maintainability, and can help prevent accidental changes to important values in your applications.

PHP: Constants