Compare define() vs const in PHP

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.