Constants are similar to variables (as they are used to store some data) but they have a fixed value: you cannot modify a constant after you have defined it.

In PHP constants can be defined by using the keyword define. It's considered good practice to use uppercase names for constants.

define('CONSTANT_NAME', 'CONSTANT_VALUE');

<?php #constants.php
     //defining a constant
     define("USERNAME", "John");

     //using the constant
     echo "<p>Hello, <strong>" . USERNAME . "</strong>.</p>";

    /*
        Hello, John.
   */
?>

Click here to read about if-else statements