What are the best practices for separating server-side and client-side code in web development?

To separate server-side and client-side code in web development, it is best practice to keep the server-side code (such as PHP) separate from the client-side code (such as JavaScript) to improve code organization, maintainability, and security. One way to achieve this separation is by creating separate files for server-side and client-side code and using AJAX requests to communicate between them.

// server-side code in PHP file
<?php
// perform server-side logic here

// return data to client-side code
echo json_encode($data);
?>

// client-side code in JavaScript file
$.ajax({
  url: 'server-side-script.php',
  method: 'GET',
  success: function(data) {
    // handle data returned from server-side script
  },
  error: function() {
    // handle errors
  }
});