Hey guys,
I'm trying to figure out how to add a BCC to my email form.

Can someone help me out?

PHP Code:

<?php

$where_form_is="http://".$_SERVER['SERVER_NAME'].strrev(strstr(strrev($_SERVER['PHP_SELF']),"/"));

session_start();
if( ($_SESSION['security_code']==$_POST['security_code']) && (!empty($_POST['security_code'])) ) { 
mail("myemail@email.com","2008 MACYAC Registration","Form data:

First Name: " . $_POST['field_1'] . " 
Last Name: " . $_POST['field_2'] . " 
Email address: " . $_POST['field_3'] . " 
Phone Number: " . $_POST['field_4'] . " 
Address : " . $_POST['field_5'] . " 
City: " . $_POST['field_6'] . " 
State : " . $_POST['field_7'] . " 
Zip Code: " . $_POST['field_8'] . " 
Birthdate: " . $_POST['field_9'] . " 
Shirt Size : " . $_POST['field_10'] . " 
Roommate Preference 1: " . $_POST['field_11'] . " 
Roommate Preference 2: " . $_POST['field_12'] . " 
Your Church: " . $_POST['field_13'] . " 
Other Church: " . $_POST['field_14'] . " 
Emergency Contact : " . $_POST['field_15'] . " 
Emergency Phone Number: " . $_POST['field_16'] . " 
Insurance Company: " . $_POST['field_17'] . " 
Insurance Policy ID: " . $_POST['field_18'] . " 



");

include("confirm.html");
}
else {
echo "Invalid Captcha String.";
}

?>

Search for "BCC" on http://us.php.net/manual/en/function.mail.php . It's right there in an example (hint #4).

I've been googling it the past hour and from what I understand, I need to add this line:

PHP Code:

$headers .= 'Bcc: birthdaycheck@example.com' . "\r\n"; 


But I'm not sure where.

Do i put the header after the "message"?

anyone?

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

How would I use this with the code above? I think it's done a little differently.

How would I use this with the code above? I think it's done a little differently.

Nope.
Your problem might be that you use a "physical" return, rather than an explicit \r\n to spearate your headers.
And for your question, simply add the BCC in the headers, before the mail body. Usually near the To: or From:
http://www.php.net/manual/en/function.mail.php

Description

bool mail ( string $to , string $subject , string $message [, string $additional_headers [, string $additional_parameters ]] )

additional_headers (optional) String to be inserted at the end of the email header.
This is typically used to add extra headers (From, Cc, and Bcc). Multiple extra headers should be separated with a CRLF (\r\n).

Note: When sending mail, the mail must contain a From header. This can be set with the additional_headers parameter, or a default can be set in php.ini.
Failing to do this will result in an error message similar to Warning: mail(): "sendmail_from" not set in php.ini or custom "From:" header missing. The From header sets also Return-Path under Windows.

Note: If messages are not received, try using a LF (\n) only. Some poor quality Unix mail transfer agents replace LF by CRLF automatically (which leads to doubling CR if CRLF is used). This should be a last resort, as it does not comply with » RFC 2822.