Look at the difference in single quotes/double quotes. You use double quotes to surrond the query, but try to concatenate with single quotes.
// Current
$query = mysql_query("Select username FROM $table WHERE username = "'.$_SESSION['username'].'" order by date DESC",$db);
// Correct
$query = mysql_query("Select username FROM $table WHERE username = '".$_SESSION['username']."' order by date DESC",$db);