PHP Constants

A constant is an identifier (name) for a simple value that cannot be changed during script execution. Unlike variables, constants do not use the $ prefix and are global by default.

Defining Constants with define()

The define() function creates a constant at runtime. It accepts the name, value, and an optional case-insensitive flag (deprecated in PHP 8).

define() Function

<?php
define("SITE_NAME", "Tutorials Logic");
define("MAX_USERS", 1000);
define("PI", 3.14159);

echo SITE_NAME;   // Tutorials Logic
echo MAX_USERS;   // 1000
echo PI;          // 3.14159

// Constants are global — accessible inside functions too
function showSite() {
    echo SITE_NAME; // works without global keyword
}
showSite();
?>

Defining Constants with const

The const keyword defines constants at compile time. It can be used inside classes and is preferred for class-level constants.

const Keyword

<?php
const APP_VERSION = "2.5.0";
const DEBUG_MODE  = false;

echo APP_VERSION; // 2.5.0

// Inside a class
class Config {
    const DB_HOST = "localhost";
    const DB_PORT = 3306;
}

echo Config::DB_HOST; // localhost
echo Config::DB_PORT; // 3306

// Retrieve constant value dynamically
$name = "APP_VERSION";
echo constant($name); // 2.5.0
?>

Predefined Constants

PHP ships with many built-in constants you can use anywhere in your scripts.

Predefined Constants

<?php
echo PHP_VERSION;    // e.g. 8.2.0
echo PHP_INT_MAX;    // 9223372036854775807
echo PHP_INT_MIN;    // -9223372036854775808
echo PHP_FLOAT_MAX;  // 1.7976931348623E+308
echo PHP_EOL;        // newline character (\n on Linux)
echo PHP_MAJOR_VERSION; // e.g. 8
echo DIRECTORY_SEPARATOR; // / on Linux, \ on Windows

// Magic constants (change based on where they are used)
echo __FILE__;   // full path to current file
echo __DIR__;    // directory of current file
echo __LINE__;   // current line number
echo __FUNCTION__; // current function name (inside a function)
echo __CLASS__;    // current class name (inside a class)
?>

Magic Constants in Context

Magic constants are special constants whose values change depending on where they are used.

Magic Constants

<?php
function greet() {
    echo "Function: " . __FUNCTION__; // greet
}
greet();

class Animal {
    public function speak() {
        echo "Class: " . __CLASS__;   // Animal
        echo "Method: " . __METHOD__; // Animal::speak
    }
}

$a = new Animal();
$a->speak();

// Namespace magic constant
namespace MyApp;
echo __NAMESPACE__; // MyApp
?>

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PHP Constants

Defining Constants with define()

Defining Constants with const

Predefined Constants

Magic Constants in Context

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