U need to pass username using SESSION variable for the same
write a simple sql query to get the username from any variable you are taking from user to make sure that the particular user is the correct user.i am taking password.
$query = "SELECT name FROM users WHERE password='$password'";
$username = mysql_result(mysql_query($query),0);
$_SESSION['username'] = $username;
than proceed as you are doing
<?php
session_start();
if(isset($_SESSION['login']) && isset($_SESSION['username']))
{
echo "logged in as".$_SESSION['username'];
}