How to fetch Email addres of an author via ID?

I want to make an HTML form which will have an input field to fill in the ID of the user. Then I want to pass the value into a php code such that the email address associated with that user ID is displayed. However, I am new to WP and don’t know how to do it correctly.

I made a new php file in the root folder of my WP directory and put in this code and tried by browsing the file, but it didn’t work.

<?php $  field = 53; $  value = 53; function get_user_by( $  field, $  value ) { $  userdata = WP_User::get_data_by( $  field, $  value );  if ( ! $  userdata ) {     return false; }  $  user = new WP_User; $  user->init( $  userdata );  var_dump($  user); } ?> 

I searched in the web as well, but couldn’t find anything of this sort? Can someone guide me through this?