How can PHP developers handle usernames with special characters or umlauts while still maintaining security and functionality?

When handling usernames with special characters or umlauts in PHP, developers should sanitize and validate input to prevent security vulnerabilities such as SQL injection or cross-site scripting attacks. One way to handle this is by using PHP's `filter_var` function with the `FILTER_SANITIZE_STRING` filter to remove any unwanted characters and ensure the username is safe to use in the application.

$username = filter_var($_POST['username'], FILTER_SANITIZE_STRING);