In the context of PHP mail() function usage, what are the potential consequences of undefined variables like $subject in the code snippet provided?

Undefined variables like $subject in the code snippet provided can lead to errors or unexpected behavior when trying to send an email using the mail() function. To solve this issue, you should always ensure that variables used in the mail() function are defined and have appropriate values assigned to them before calling the function.

// Check if $subject is defined and assign a default value if not
if (!isset($subject)) {
    $subject = "Default Subject";
}

// Use the $subject variable in the mail() function
mail($to, $subject, $message, $headers);