What are the potential issues with including login_einfach.php in multiple files?

Including login_einfach.php in multiple files can lead to conflicts if the file contains functions or variable declarations that are already defined in the including files. To solve this issue, you can use PHP's `include_once` or `require_once` functions to ensure that the file is only included once throughout the execution of the script.

// Include login_einfach.php only once
require_once 'login_einfach.php';