https://forums.pocketmine.net/threads/creating-a-timer-with-tasks.11098/ Run a task every few ticks to sendTip to the player
PHP: $player->sendTip(TextFormat::GREEN.str_repeat('|',$tick).TextFormat::RED.str_repeat('|',$max - $tick)); where $tick is the current tick and $max is the max number of '|' you want to see