Assuming your are using the standard mail() function:
$contact_email = "whoever@whatever.com, whoever2@whatever.com";
Or if you actually want to send a blind copy:
$recipient = "whoever@whatever.com";
$subject = "Whatever";
$message_body = "whassup?";
$headers = "From: \"Joe Blow\" <jblow@abc.com>\n";
$headers .= "Bcc: whoever2@whatever.com\n";
mail($recipient, $subject, $message_body, $headers);