How do I check user input against data in a table in php?

Question

If the user inputs data in form field 'AddGuestName' i would like to query 'a_table' to see if they are allowed an additional guest. The column 'addguest' contains a "Y" if they can have an additional guest and a "N" if they cannot. If they cannot i would like to display and error message and not save the data until the 'AddGuestName' field is blank. Not sure where im going wrong.

My Code

$Name = check_name($_POST['name'],
 " Enter Your First and Last Name....Click Your Browsers Back Button To Re-Enter ");    

$RsvpNum = check_input($_POST['RsvpNum'], " Enter The 3 Digit RSVP Number Located On Your RSVP Card....Click Your Browsers Back Button To Re-Enter ");

$AddGuestName = check_rsvp($_POST['AddGuestName'], "You Are Not Allowed An Additional Guest.....Click your Browsers Back Button To Re-Enter");


function check_rsvp($data, $query, $result, $RsvpNum, $AddGuestName, $problem='')<br />
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
   if ($problem && strlen($data) > 0)
    {
mysql_query($query="SELECT addguest FROM a_table WHERE rsvp_num=$RsvpNum");
    $result=mysql_query($query);
    }
    return $data;



    if (($result) == 'N')
        {
            die($problem);
        }
        return $data;
    }

Answer 1

Are you meaning to return $data right after that first if ? I'm thinking no...

Answer 2

$query="SELECT addguest FROM guests WHERE rsvp_num=$RsvpNum LIMIT 1";
$result=mysql_query($query);
$row = mysql_fetch_row($result);

if ($row[0] =='y'){
 ...