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PHP - Constants

Introduction to Constants​

A constant in PHP is a name or identifier for a value that remains unchanged during the execution of the script. Constants are case-sensitive by default and are typically named using uppercase letters.

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 names: 
    define("2TWO", "second thing");

Difference between Constants and Variables​

  • Constants are defined using define() and cannot be redefined or undefined.
  • Constants have global scope and can be accessed anywhere in the script.

Defining a Named Constant​

The define() function is used to define constants in PHP.

define(string $const_name, mixed $value, bool $case = false): bool
  • $const_name: The name of the constant.
  • $value: The value of the constant (scalar or array).
  • $case: Optional. If set to true, the constant will be case-insensitive.

Example​

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

   echo CONSTANT; // Outputs: Hello world.
   // echo Constant; // Generates an error
?>

Using the constant() Function​

You can also use the constant() function to retrieve the value of a constant.

constant(string $name): mixed

Example​

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

   echo MINSIZE; // Outputs: 50
   echo constant("MINSIZE"); // Outputs: 50
?>

Using the defined() Function​

The defined() function checks if a constant is defined.

defined(string $name): bool

Example​

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

   if (defined('MAX')) {
      echo MAX; // Outputs: 100
   }
?>

Getting All Defined Constants​

PHP provides the get_defined_constants() function to retrieve all defined constants.

Example​

<?php
   $constants = get_defined_constants(true);
   echo "<pre>";
   print_r($constants);
   echo "</pre>";
?>

This will display an associative array of all defined constants and their values.

This concludes the overview of constants in PHP.