ช่วยอธิบายโค๊ดนี้หน่อยครับ Add/Update/Delete จะเอาเป็น Code ตัวอย่างดีไหมครับ

<?php  

function db_connect() {  
  $database_name = 'xxxxxxx'; // Set this to your Database Name  
  $database_username = 'xxxxxxx'; // Set this to your MySQL username  
  $database_password = 'xxxxxxx'; // Set this to your MySQL password  
  $result = mysql_pconnect('localhost',$database_username, $database_password);  
  if (!$result) return false;  
  if (!mysql_select_db($database_name)) return false;  
  return $result;  
}  

$conn = db_connect(); // Connect to database  
$query; 
$type = isset($_REQUEST['pr']) ? $_REQUEST['pr'] : ''; 
$id = isset($_REQUEST['id']) ? $_REQUEST['id'] : ''; 
$fn = isset($_REQUEST['fn']) ? $_REQUEST['fn'] : ''; 
$ln = isset($_REQUEST['ln']) ? $_REQUEST['ln'] : ''; 
$ad = isset($_REQUEST['ad']) ? $_REQUEST['ad'] : ''; 
$ct = isset($_REQUEST['ct']) ? $_REQUEST['ct'] : ''; 
$st = isset($_REQUEST['st']) ? $_REQUEST['st'] : ''; 
$zp = isset($_REQUEST['zp']) ? $_REQUEST['zp'] : ''; 
$cn = isset($_REQUEST['cn']) ? $_REQUEST['cn'] : ''; 
$ph = isset($_REQUEST['ph']) ? $_REQUEST['ph'] : ''; 

if ( $type == "Update") { 
  $query = "UPDATE GUEST SET " 
          . "F_NAME = '" . $fn 
          . "', L_NAME = '" . $ln 
          . "', STR_ADDRESS = '" . $ad 
          . "', CITY = '" . $ct 
          . "', STATE = '" . $st 
          . "', ZIP = " . $zp 
          . ", COUNTRY = '" . $cn 
          . "', PHONE_Num = '" . $ph 
          . "' WHERE  GUEST_ID = " . $id; 
} 
if ( $type == "Add") { 
  $query = "INSERT INTO GUEST " 
          . "(F_NAME, L_NAME, STR_ADDRESS, " 
          . "CITY, STATE, ZIP, COUNTRY, PHONE_Num)" 
          . " VALUES ("   
          . "'"    . $fn . "', '" . $ln 
          . "', '" . $ad . "', '" . $ct 
          . "', '" . $st . "', "  . $zp 
          . ", '"  . $cn . "', '" . $ph . "')"; 
} 
if ( $type == "Delete") { 
  $query = "DELETE FROM GUEST WHERE  GUEST_ID = " . $id; 
} 
echo $query; 
if ($conn && $query != "") {  
  $result = mysql_query($query,$conn); 
} 
?>