časově neomezená session nebo session co vyprší po 20dnech

<?php
ob_start();
$host="localhost"; 
$username=""; 
$password="";
$db_name="test"; 
$tbl_name="members"; 

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

$myusername=$_POST['myusername']; 
$mypassword=$_POST['mypassword']; 

$myusername = stripslashes($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);

$count=mysql_num_rows($result);


if($count==1){
session_register("myusername");
session_register("mypassword"); 
header("location:login_success.php");
}
else {
echo "Wrong Username or Password";
}

ob_end_flush();
?>