Code to update Data in MYSQL using PHP

$host="localhost"; // Host name 
$db_username="root"; // Mysql username 
$db_password=""; // Mysql password 
$db_name="sarvdev"; // Database name 
$user="user"; // Table names 


mysql_connect("$host", "$db_username", "$db_password")or die("cannot connect"); 
mysql_select_db("$db_name")or die("cannot select DB");


$sql="Update user set name=$_POST['txtname'],age=$_POST['ddlage'] where id='" . $userid . "'";
$result=mysql_query($sql);
?>
Here txtname and ddlage is controls name from where data is coming.