You do not want to create an infinite loop, but you can set a timeout. Basically, a loop for X-second waits for some data, and if this does not happen, send a response to the client stating that he needs to initiate a new request that will have the same timeout period.
$source; // some data source - db, etc $data = null; // our return data $timeout = 30; // timeout in seconds $now = time(); // start time // loop for $timeout seconds from $now until we get $data while((time() - $now) < $timeout) { // fetch $data $data = $source->getData(); // if we got $data, break the loop if (!empty($data)) break; // wait 1 sec to check for new $data usleep(10000); } // if there is no $data, tell the client to re-request (arbitrary status message) if (empty($data)) $data = array('status'=>'no-data'); // send $data response to client echo json_encode($data);