PHP Fatal error: [] operator not supported for strings

gistfile1.php

<?php

 $dbhost="localhost";
    $dblogin="house";
    $dbpwd='password';
    $dbname="house";
    $db =  mysql_connect($dbhost,$dblogin,$dbpwd);
    mysql_select_db($dbname);


  // 1.  Get all of the sensors.  ID, Name, Serial Number? (correctly associate Name => Number)
  // 2.  Get all of the times for $date
  // 3.  Get sensorname, temperature where date = $date
  // 4.  Hash of hashes!
  //   $data = [
  //     '12:01' = [ 
  //       'Workshop' = 79,
  //       'Computer Room' = 80,
  //       'Kitchen' = 81
  //     ],
  //     '12:02' = [ 
  //       'Workshop' = 80,
  //       'Computer Room' = 82,
  //       'Kitchen' = 78
  //     ],


  // Every distinct hour
  $query_time = "select distinct concat(time_format(from_unixtime(statDate), '%h'), time_format(from_unixtime(statDate), '%p')) as Date from stats where date(from_unixtime(statDate)) = curdate()";
  $result_time = mysql_query($query_time) or die ('query failed: ' . mysql_error());
  while ($time = mysql_fetch_array($result_time)) {
    $times[] = $time;
  }

  // Find all of the sensors
  $query_rooms = 'select distinct SensorID, SensorName from sensors';
  $result_rooms = mysql_query($query_rooms) or die ('query failed: ' . mysql_error());
  // Put all of the sensors from the above query into an array
  while ($row = mysql_fetch_array($result_rooms)) {
    $rooms[$row[0]] = $row[1];
  }

  while ($room = current($rooms)) {
    $$room = $room;
    $roomid =  key($rooms);
    $query = "select statValue from stats where date(from_unixtime(statDate)) = curdate() AND SensorID = $roomid order by StatDate ASC";
    $result = mysql_query($query) or die('Query failed: ' . mysql_error());
    while ($row = mysql_fetch_array($result)){
      ${$room}[] = $row[0];
    }
    next($rooms);
  }
  mysql_close($db);

?>