How can a beginner ensure that POST data is properly sanitized before using it in a mail function in PHP?

To ensure that POST data is properly sanitized before using it in a mail function in PHP, beginners can use the `filter_var` function with the `FILTER_SANITIZE_STRING` filter to remove any potentially harmful characters from the input data. Additionally, they can use the `htmlspecialchars` function to encode special characters to prevent cross-site scripting attacks.

// Sanitize POST data
$email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);
$message = htmlspecialchars($_POST['message']);

// Use sanitized data in mail function
$to = 'recipient@example.com';
$subject = 'Message from website';
$headers = 'From: ' . $email;

mail($to, $subject, $message, $headers);