PHP - Constant Arrays

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In PHP, an array is a collection of elements that can have several values under a single name. Normally, arrays in PHP are mutable, which means you can add, remove or change elements after they are generated. Constant arrays, on the other hand, are useful when you want an array with values that never change during your program.

A constant array is one that cannot be altered once defined. You have access to its components, but you cannot modify, add or remove them. Prior to PHP 5.6, it was not possible to declare constant arrays; however, current versions of PHP introduced simple methods to perform this using the const keyword or the define() function.

Unlike a normal array, its identifier doesn't start with the "$" sign.

Syntax of the Constant Arrays

The syntax for declaring constant array using const is −

const ARRAY_NAME = [
   "val1", 
   "val2", 
   "val3"
];

The syntax for defining constant array using define() is −

define('ARRAY_NAME', [
   "val1", 
   "val2", 
   "val3"
]);

Constant Array with const

In the below example we have used const to define the constant array and displayed using the var_dump() function.

<?php
   const FRUITS = array(
      "Watermelon", 
      "Strawberries",
      "Pomegranate",
      "Blackberry",
   );
   var_dump(FRUITS);
?>

Output

It will generate the following output −

array(4) {
   [0]=>
   string(10) "Watermelon"
   [1]=>
   string(12) "Strawberries"
   [2]=>
   string(11) "Pomegranate"
   [3]=>
   string(10) "Blackberry"
}

You can also use the conventional square bracket syntax to declare a constant array in PHP −

const FRUITS = [
   "Watermelon", 
   "Strawberries",
   "Pomegranate",
   "Blackberry",
];

Try to Modify the Constant Array

It is not possible to modify any element in a constant array. Hence, the following code throws a fatal error −

<?php
   const FRUITS = [
      "Watermelon", 
      "Strawberries",
      "Pomegranate",
      "Blackberry",
   ];
   FRUITS[1] = "Mango";
?>

Output

It will produce the following output −

PHP Fatal error:  Cannot use temporary expression in write context

Constant Arrays PHP 7 Onwards (With define())

The newer versions of PHP allow you to declare a constant array with define() function.

<?php
   define ('FRUITS',  [
      "Watermelon", 
      "Strawberries",
      "Pomegranate",
      "Blackberry",
   ]);
   print_r(FRUITS);
?>

Output

It will produce the following output −

Array
(
   [0] => Watermelon
   [1] => Strawberries
   [2] => Pomegranate
   [3] => Blackberry
)

You can also use the array() function to declare the constant array here.

define ('FRUITS',  array(
   "Watermelon", 
   "Strawberries",
   "Pomegranate",
   "Blackberry",
));

Associative Constant Array with define()

It is also possible to declare an associative constant array. Here is an example −

<?php
   define ('CAPITALS',  array(
      "Maharashtra" => "Mumbai",
      "Telangana" => "Hyderabad",
      "Gujarat" => "Gandhinagar",
      "Bihar" => "Patna"
   ));
   print_r(CAPITALS);
?>

Output

It will produce the following output −

Array
(
   [Maharashtra] => Mumbai
   [Telangana] => Hyderabad
   [Gujarat] => Gandhinagar
   [Bihar] => Patna
)