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PHP Constants

In the real world, a constant is a thing that does not change

Same theory in the programming world, constant is a piece of information stored in the computer's memory which does not change (cannot be changed).

PHP Constants

  • Once a constant is defined, it cannot be changed.
  • All the constants have global scopes, even it is defined inside a function.
  • Unlike variables, constants do not have $ before the name.
  • A constant name should start with a letter or underscore.
  • A constant name can have letters, numbers, ASCII characters between 127-255 after the first letter.

PHP Declaring Constants

PHP does not have a keyword to declare a constant. We have to use define() function for the purpose.

define() Syntax

define(name, value, case-insensitive)


  • name: The name of the constant
  • value: The value of the constant - only boolean, integer, float, and string prior to 5.6. Arrays can be used from PHP 5.6 onwards. (See data types chapter to learn about booleans, integers, floats, strings, and arrays.)
  • case-insensitive: Specify whether the constant is case-insensitive or not. The default is false (case-sensitive).

PHP Constants Example

// Valid constant names
define("GREETING", "Hello World");
define("GREETING2", "Hello You");
define("GREETING_3", "Hello everyone");

// Invalid constant names
define("2GREETING", "Hello");

echo GREETING, '<br>';
echo GREETING2, '<br>';
echo GREETING_3, '<br>';

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All of above constants are case sensitive

PHP Constants Case-Insensitive Example

define('GREETING', 'Hello World', true);

echo GREETING, '<br>';
echo Greeting, '<br>';
echo gReeting, '<br>';

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Here, all GREETING, Greeting, and gReeting is equal because the constant is case-insensitive.

Constants are Global

All the constants have global scopes. We can use constants within the functions directly.

PHP Constants are Global Example

define('GREETING', 'Hello World');

function hello() {


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You can even define constants inside functions and use them outside of it.

PHP Constants: Defining Inside Functions Example

function defineConstant() {
	define('GREETING', 'Hello World');

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