$username = "JohN_Doe123"; $formatted_username = format_username($username); echo $formatted_username; // john_doe
$username = "Sam"; $formatted_username = format_username($username); echo $formatted_username; // user_samThese examples show how the function can format a username with different input styles and prefix with "user_" if the username is too short. The package library for this function cannot be determined as it depends on the implementation.