PHP Constants

by Vincy. Last modified on July 1st, 2022.

Like as many programming languages, PHP also contains constants. It has some predefined constants in its core like PHP_VERSION, PHP_DEBUG and more. We can define constants by using the define() function.

Generally, constants are in the global scope. We can also define class-specific or interface-specific constants by using the PHP const keyword. In this tutorial, we are going to see examples for defining and accessing constants.

PHP constants are case-sensitive by default. We can enable case-insensitivity while defining constants. Conventionally, constants should have uppercase letters and should start with the underscore ( _ ) or a letter.

In a previous tutorial, we have seen PHP variables which are completely differed from constants. PHP contains some special constants named magic constants.

These magic constants are not actually constants because of revealing different values based on the place they are used. The magic constants are case-insensitive.

Defining PHP constants using define()

This code defines the PI constant and prints it to the browser using a PHP echo statement.

define("PI", 3.14);
echo PI; // Output: 3.14

In the above program, the constant is defined as a key-value pair. The key is used to get the value of the constant.

In the above program, the define() function gets only two parameters. The define() has an optional third parameter to set the case insensitivity of a constant. By default, the constants are case-sensitive.

We can make them case insensitive by setting the case-insensitivity flag to true. So, the above example code can be changed as,

define("PI", 3.14, true);
echo pi; // Output: 3.14

PHP Constants with const

PHP const is a language construct where the define() is a function. We should use static name and value with the const statement to initialize constants where the define() function allows variable interpolation and function call while defining constant. Constants defined using this construct are always case-sensitive.

Defining constants using the const statement is good in the readability aspect, though the define() has many features by enabling case insensitivity, allowing function calls assigning values to the constant, and revealing constants at runtime.

The following line shows the example of initializing constants using a const statement.


const PI = "3.14";
echo PI; // Output: 3.14

Related Functions to Access Constants


PHP constant() function is used to get the value of the constant by sending its id as an argument of this function. The following example code uses the constant() function to get the constant value by using the id.

define("PI", 3.14);
define("USERTYPE", "GUEST");
$id = "PI";
echo constant($constant);


The get_defined_constants() function will dump all user-defined constants and core constants into an array by default. It receives a boolean value as its argument to categorize the constants.

If it is set to true, then this function will categorize all built-in constants and user-defined constants with a multidimensional array.

Written by Vincy, a web developer with 15+ years of experience and a Masters degree in Computer Science. She specializes in building modern, lightweight websites using PHP, JavaScript, React, and related technologies. Phppot helps you in mastering web development through over a decade of publishing quality tutorials.

Comments to “PHP Constants”

  • Nicolás says:

    $id = “PI”;
    echo constant($constant);

    In this example shouldn’t constant() receive $id? And i dont understand whats the use of the second define()

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