Help with a PLUGIN!!

PHP:

<?php/*
__PocketMine Plugin__
name=OnlineCounter
version=1.0
author=JassperBeastHD
class=counter
apiversion=10
*/class counter implements Plugin{
private $api;
public function __construct(ServerAPI $api, $server = false){$this->all = $api;
}

public function init(){$this->api->schedule(400, array($this, "counter"), array(), true);
}

public function __destruct(){}

public function counter(){$output .= "There are ".count($this->server->clients)."/".$this->server->maxClients." players online:\n";
 if(count($this->server->clients) == 0){
 break;
 }$this->api->chat->broadcast($output);
 }
}

This give the error!

You haven't defined $this->server.
Also you need to put $this->api = $api; in the __construct function.