$first_level = mysql_query("SELECT id, email FROM $db[users] WHERE parent_uid='$uid'");
while($f = mysql_fetch_array($first_level)) {
	echo "Level 1: $f[id] $f[email]<BR>";
	$second_level = mysql_query("SELECT id,email FROM $db[users] WHERE parent_uid='$f[id]'");
	while($s = mysql_fetch_array($second_level)) {
		echo "&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Level 2: $s[id] $s[email]<BR>";
		$third_level = mysql_query("SELECT id, email FROM $db[users] WHERE parent_uid='$s[id]'");
		while($t = mysql_fetch_array($third_level)) {
			echo "&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Level 3: $t[id] $t[email]<BR>";
		}
	}

	echo "------------<BR>";
}


Output:

Level 1: 4005 fk1wa8j2us@test.dailyclassifieds.com
------------
Level 1: 4004 hu5v2v8cgq@test.dailyclassifieds.com
     Level 2: 4002 xl6ez99by3@test.dailyclassifieds.com
          Level 3: 4000 xe4leuarcp@test.dailyclassifieds.com
          Level 3: 3999 t8kyd8qc82@test.dailyclassifieds.com
          Level 3: 3998 nca1vag4fs@test.dailyclassifieds.com
          Level 3: 3997 9z6849wukk@test.dailyclassifieds.com
     Level 2: 4001 216pscfx1z@test.dailyclassifieds.com
------------
Level 1: 4003 twrw4q5y41@test.dailyclassifieds.com
------------