Long polling with database data?

I have done something very similar recently. I did use jQuery .ajax call instead of the generic XMLhttprequest but the idea is the same:

recentFunction(container, lastDate){    var lastDate = "";    return $.ajax({        type: "POST",        url: "getData.php",        cache: false,        data: { 'request': 'recent',            'param': lastDate },        dataType: "json",        success: function(data){            if(data != null){                $.each(data, function(key, value){                    if(key == 0){                        lastDate = value['date_added'];                    }                    var html = "some html here";                    // append html to dom element here                                // and delete any old items here if needed                });            }        },        complete: function(){            setTimeout(function(){recentFunction(container, lastDate)}, 7000);        }    });}

in the getData.php file I have query with a where clause that gets any items from db that are more recent than last element. Default value for $lastDate is set to 0, so it returns all items if no date is submitted.

<?php$lastDate = 0;$recent = array();$recentQuery = "SELECT id, date FROM someTable WHERE date > '" . $lastDate . "'";$recentResults = $db->query($recentQuery);while($r = $recentResults->fetch_array(MYSQLI_ASSOC)){      $recentMovies[] = $r;}echo json_encode($recentMovies);?>