PHP - Constants

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A constant in PHP is a name or an identifier for a simple value. A constant value cannot change during the execution of the PHP script. It basically improves code readability and maintainability. Constants make it easier to manage the configuration settings.

They make code easier to read and prevent key values from changing. Constants can be used anywhere in the application. PHP also contains built-in constants such as PHP_VERSION. Constants help make the code simple and secure.

Rules of Constants in PHP

• By default, a PHP constant is case-sensitive.

• By convention, constant identifiers are always uppercase.

• A constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscore.

• There is no need to write a dollar sign ($) before a constant, however one has to use a dollar sign before a variable.

How to Define a Constant in PHP?

We can use the define() function to create a constant in PHP programming language.

Syntax

Below is the syntax of the define() function in PHP −

   define("CONSTANT_NAME", value);

Parameters

The parameters are needed to use the define() function are mentioned below −

Rules for Defining Constants

Here are the rules listed below for defining a constant −

• A constant name must start with a letter or underscore.

• Unlike variables, a constant name should not contain the $ sign.

• Constants are global, so they can be accessed from anywhere in the script.

Examples of Valid and Invalid Constant Names in PHP

Here are some examples of valid and invalid constant names in PHP −

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

// Invalid constant names
define("2TWO",    "second thing");

PHP Predefined Constants

PHP has several built-in constants which provide useful information.

<?php
   // Provides the current PHP version
   echo PHP_VERSION; 
   
   // Provides the operating system PHP is running on
   echo PHP_OS; 
?>

Defining a Named Constant

The define() function in PHP library is used to define a named constant at runtime.

define(string $const_name, mixed $value, bool $case = false): bool

Parameters

• const_name − The name of the constant.

• value − The value of the constant. It can be a scalar value (int, float, string, bool, or null) or array values are also accepted.

• case − If set to true, the constant will be defined case-insensitive. The default behavior is case-sensitive, i.e., CONSTANT and Constant represent different values.

The define() function returns "true" on success and "false" on failure.

Example 1

The following example demonstrates how the define() function works −

<?php  
   define("CONSTANT", "Hello world.");

   echo CONSTANT; 
   // echo Constant; 
?>

Output

The first echo statement outputs the value of CONSTANT. You will get the following output −

Hello world.

But, when you uncomment the second echo statement, it will display the following error −

Fatal error: Uncaught Error: Undefined constant "Constant" in hello.php: on line 5

If you set the case parameter to False, PHP doesn't differentiate upper and lowercase constants.

Example 2

You can also use an array as the value of a constant. Take a look at the following example −

<?php  
   define(
      $name="LANGS", 
      $value=array('PHP', 'Java', 'Python')
   );
   var_dump(LANGS);
?>

Output

It will produce the following output −

array(3) {
  [0]=>
  string(3) "PHP"
  [1]=>
  string(4) "Java"
  [2]=>
  string(6) "Python"
}

Using the constant() Function

The echo statement outputs the value of the defined constant. You can also use the constant() function. It returns the value of the constant indicated by name.

constant(string $name): mixed

The constant() function 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.

<?php
   define("MINSIZE", 50);

   echo MINSIZE;
   echo PHP_EOL;
   // same thing as the previous line
   echo constant("MINSIZE");	
?>

Output

It will produce the following output −

50
50

Using the defined() Function

The PHP library provides a defined() function that checks whether a given named constant exists. Take a look at the following example −

<?php
   define('MAX', 100);

   if (defined('MAX')) {
      echo MAX;
   }
?>

Output

It will produce the following output −

100

PHP also has a function called "get_defined_constants()" that returns an associative array of all the defined constants and their values.

Difference between Constants and Variables in PHP

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

• Constants may be defined and accessed anywhere without regard to variable scoping rules.

• Once the Constants have been set, they may not be redefined or undefined.