Compare define() vs const in PHP

In PHP, both define() and const are used to declare constants, but they have important differences in when and how they can be used.

Syntax

<?php
    const VAR = 'FOO';
    define('VAR2', 'BAR');
    
    echo VAR . "<br>";
    echo VAR2;
?>

FOO
BAR

Key Differences

Compile-time vs Runtime Definition

The basic difference is that const defines constants at compile time, whereas define() defines them at run time ?

<?php
    // This works - const is defined at compile time
    const COMPILE_TIME = 'Available immediately';
    
    // This also works - define() at runtime
    define('RUNTIME', 'Available after execution');
    
    echo COMPILE_TIME . "<br>";
    echo RUNTIME;
?>

Available immediately
Available after execution

Conditional Definition

We can't use const in conditional blocks, while define() allows conditional constant declaration ?

<?php
    $condition = true;
    
    if ($condition) {
        // const VAR = 'FOO'; // This would cause a syntax error
        define('CONDITIONAL_CONST', 'This works');
    }
    
    echo CONDITIONAL_CONST;
?>

This works

Value Types

const accepts only static scalars (strings, numbers, booleans, null), whereas define() accepts any expression ?

<?php
    // const with static values
    const STATIC_VALUE = 'Hello';
    const NUMBER_VALUE = 42;
    
    // define() can use expressions
    define('EXPRESSION_VALUE', 'Hello ' . 'World');
    define('CALCULATED', 10 * 5);
    
    echo STATIC_VALUE . "<br>";
    echo EXPRESSION_VALUE . "<br>";
    echo CALCULATED;
?>

Hello
Hello World
50

Case Sensitivity

const is always case-sensitive, while define() can optionally be case-insensitive ?

<?php
    const CASE_SENSITIVE = 'Always case sensitive';
    define('CASE_INSENSITIVE', 'Can be case insensitive', true);
    
    echo CASE_SENSITIVE . "<br>";
    echo case_insensitive; // Works due to third parameter being true
?>

Always case sensitive
Can be case insensitive

Class Constants

const can be used inside classes to declare class constants, but define() cannot ?

<?php
    class MyClass {
        const CLASS_CONSTANT = 'Valid class constant';
        // define('INVALID', 'value'); // This would cause an error
        
        public function showConstant() {
            return self::CLASS_CONSTANT;
        }
    }
    
    echo MyClass::CLASS_CONSTANT . "<br>";
    $obj = new MyClass();
    echo $obj->showConstant();
?>

Valid class constant
Valid class constant

Comparison Summary

Conclusion

Use const for simple, static constants and class constants. Use define() when you need conditional definition, expressions, or case-insensitive constants.