It would be more efficient to identify nether portals when one is created rather than to identify the frame every time the player moves. Therefore, you are advised only to check when the player collide with a portal block but not to check the frames in move event. In addition, check portal creation when fire charge or flint&steel is used on an obeidian.
You can use the PlayerMoveEvent and find out if he's standing inside a block (Nether Portal Block): PHP: public function onMove(PlayerMoveEvent $event) { $player = $event->getPlayer(); if($player->getLevel()->getBlock($player)->getId() === 90) { // if player stands inside Nether Portal Block // do something }}