PHP - Constants

A constant is an identifier (name) for a simple value. A constant value cannot change during the execution of the script. Constants are case-sensitive. By convention, constant identifiers are always uppercase.

The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

Note: Like superglobals, the scope of a constant is global. Constants can be accessed from anywhere in a script without regard to scope.

define() function

The PHP define() function is used to define a named constant. The Syntax for using this function is given below:

define(constant_name, value)

Parameters

Note: Prior to PHP 8.0.0, constants defined using the define() function may be case-insensitive.

constant() function

The PHP constant() function is used to get the value of the constant. It is useful if you need to retrieve the value of a constant, but do not know its name, i.e. it is stored in a variable or returned by a function. The Syntax for using this function is given below:

constant(name)

Parameters

Unlike variables, a constant does not require a $ sign. The value of a constant can be retrieved using its name. The constant() function can be used to get constant's value if it is required to obtain the constant's name dynamically.

<?php
//creating a constant
define("MAX_SIZE", 100);

//retrieving constant value
echo MAX_SIZE."\n";
//another way of retrieving constant value
echo constant("MAX_SIZE")."\n";
?>

The output of the above code will be:

100
100

Valid and invalid constant names

The name of a constant follows the same rules as any label in PHP. A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

//Valid constant names
define("FOO", "something");
define("FOO2", "something else");
define("FOO_BAR", "something more");

//Invalid constant names
define("2FOO",    "something");

Constants are Global

Like superglobals, the scope of a constant is global. Constants can be accessed from anywhere in a script without regard to scope. Consider the example below:

<?php 
function CreateConstant() {
  define("MAX_SIZE", 100);
}

CreateConstant();

//retrieving constant value
echo MAX_SIZE."\n";
?>

The output of the above code will be:

100

PHP Constant Arrays

In PHP 7, an array constant can be created. Consider the example below:

<?php
//creating an array constant
define("Colors", ["Red", "Green", "Blue"]);

//retrieving values from array constant
for ($i = 0; $i < count(Colors); $i++) {
  echo Colors[$i]."\n";
}
?>

The output of the above code will be:

Red
Green
Blue

PHP Magic constants

There are nine magical constants that change depending on where they are used. For example, the value of __LINE__ depends on the line that it's used on in your script.

All these "magical" constants are resolved at compile time, unlike regular constants, which are resolved at runtime. These special constants are case-insensitive and are as follows: