Each page request just update their last activity.
to show when they were last online, just select their lastactivity field from database and show it
where relativeTime() is function I've been using:
function relativeTime($time, $short = false){
$SECOND = 1;
$MINUTE = 60 * $SECOND;
$HOUR = 60 * $MINUTE;
$DAY = 24 * $HOUR;
$MONTH = 30 * $DAY;
$before = time() - $time;
if ($before < 0)
{
return "not yet";
}
if ($short){
if ($before < 1 * $MINUTE)
{
return ($before <5) ? "just now" : $before . " ago";
}
if ($before < 2 * $MINUTE)
{
return "1m ago";
}
if ($before < 45 * $MINUTE)
{
return floor($before / 60) . "m ago";
}
if ($before < 90 * $MINUTE)
{
return "1h ago";
}
if ($before < 24 * $HOUR)
{
return floor($before / 60 / 60). "h ago";
}
if ($before < 48 * $HOUR)
{
return "1d ago";
}
if ($before < 30 * $DAY)
{
return floor($before / 60 / 60 / 24) . "d ago";
}
if ($before < 12 * $MONTH)
{
$months = floor($before / 60 / 60 / 24 / 30);
return $months <= 1 ? "1mo ago" : $months . "mo ago";
}
else
{
$years = floor ($before / 60 / 60 / 24 / 30 / 12);
return $years <= 1 ? "1y ago" : $years."y ago";
}
}
if ($before < 1 * $MINUTE)
{
return ($before <= 1) ? "just now" : $before . " seconds ago";
}
if ($before < 2 * $MINUTE)
{
return "a minute ago";
}
if ($before < 45 * $MINUTE)
{
return floor($before / 60) . " minutes ago";
}
if ($before < 90 * $MINUTE)
{
return "an hour ago";
}
if ($before < 24 * $HOUR)
{
return (floor($before / 60 / 60) == 1 ? 'about an hour' : floor($before / 60 / 60).' hours'). " ago";
}
if ($before < 48 * $HOUR)
{
return "yesterday";
}
if ($before < 30 * $DAY)
{
return floor($before / 60 / 60 / 24) . " days ago";
}
if ($before < 12 * $MONTH)
{
$months = floor($before / 60 / 60 / 24 / 30);
return $months <= 1 ? "one month ago" : $months . " months ago";
}
else
{
$years = floor ($before / 60 / 60 / 24 / 30 / 12);
return $years <= 1 ? "one year ago" : $years." years ago";
}
return "$time";
}