How i create a Worldborder

How do I make a worldborder in my plugin that gets smaller per seconds?

 

Sounds more like a whole plugin request.

 

Can you show proof of a previous attempt?

 

PHP:

//I'm on my phone don't comment
public function denyMovement(PlayerMoveEvent $event){
Player $player = $event->getPlayer();
$x = $player->getBlockX();
$z = $player->getBlockZ();

//I don't know how to make a shrinking border so here's a basic concept by me which most likely won't work

if($x > YOURXCOORDSHERE && $z > YOURYCOORDSHERE){

$player->teleport(new Vector3($player->getWorld(), X, Y, Z);
}

}

 

well, I might get onto developing this plugin but it will require quite some time per tick.