Check Login PHP

$host=$p11server; // Host name
$username=$p11username; // Mysql username
$password=$p11password; // Mysql password
$db_name=$p11database; // Database name
$tbl_name="users"; // Table name
 
// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");
 
// username and password sent from form
$myusername=$_POST['myusername'];
$mypassword=$_POST['mypassword'];
 
// To protect MySQL injection (more detail about MySQL injection)
$myusername = stripslashes($myusername);
$mypassword = stripslashes($mypassword);
$myusername = mysql_real_escape_string($myusername);
$mypassword = mysql_real_escape_string($mypassword);
 
$sql="SELECT * FROM $tbl_name WHERE username='$myusername' and password='$mypassword'";
$result=mysql_query($sql);
 
// Mysql_num_row is counting table row
$count=mysql_num_rows($result);
// If result matched $myusername and $mypassword, table row must be 1 row
 
if($count==1){
// Register $myusername, $mypassword and redirect to correct location
session_register("myusername");
session_register("mypassword");
header("location:main.php");
}
else {
header("location:index.php?messalert=pw");
}