# Compare define() vs const in PHP

PHPProgrammingServer Side Programming

As we know both define() and const are used to declare a constant in PHP script.

## Syntax

<?php
const VAR = 'FOO'
define('VAR', 'FOO');
?>

Let's discuss the difference between these two.

• The basic difference between these two is that const defines constants at compile time, whereas define() defines them at run time.
• We can't use the const keyword to declare constant in conditional blocks, while with define() we can achieve that.
• <?php
if(){
const VAR = 'FOO'; // invalid
}
if(){
define('VAR', 'FOO'); //valid
}
?>
• const accepts a static scalar(number, string or other constants like true, false, null, __FILE__), whereas define() takes any expression.
• consts are always case sensitive, whereas define() allows you to define case insensitive constants by passing true as the third argument.

• const can also be utilized within a class or interface to declare a class constant or interface constant, while define() can't be utilized for this reason
• <?php
class abc{
const VAR = 2; // valid
echo VAR; // valid
}
// but
class xyz{
define('XUV', 2); // invalid
echo XUV;// invalid
}
?>
• The above example shows that we can declare constant inside the class with the const keyword but define() can't be used for declaring constant inside a class.
Published on 25-Jun-2019 14:53:37