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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 - boolean, integer, float, string or array. (You will learn about these in the data types chapter)
  • 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>';

Run Example ››

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>';

Run Example ››

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 functions directly.

PHP Constants are Global Example

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

function hello() {


Run Example ››

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');

Run Example ››

Why Contansts?

  • To save database credentials: Normally, we save database credentials in a single file and we will use constants to save username and passwords in. The reason for this is the value of constants cannot be changed programmatically.
    What if we need to change the password? In this case, we can simply edit our file and change the value of the contanst.
  • To save main configurations: In your website, there will be main configurations like company name, logo URL, etc. Saving these values in a constant is a good idea.
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