Have a session_start() at the top of the login page too - and in general every page where you'll ever need to get, manipulate or destroy session variables - and asign $username to $)SESSION['myusername'].
PHP Code:
<?php
session_start();
$host="localhost"; // Host name
$username="root"; // Mysql username
$password=""; // Mysql password
$db_name="realestate"; // Database name
$tbl_name="registration"; // Table name
// Connect to server and select databse.
$db=mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name", $db)or die("cannot select DB");
// username and password sent from form
global $myusername;
$myusername=$_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 file "login_success.php"
$_SESSION['myusername'] = $myusername;
$_SESSION['mypassword'] = $mypassword;
header("location:../login/adproperties success.php");
}
else {
header("refresh: 1; URL=../login/adproperties fail.php");
}
?>