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1 Answer
Alok Prasad

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.

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