Cooldown

I'm making a kit plugin and I need to know how to setup the cooldown

Use the time() function in PHP. If you have not struggled to make it yet, then please give it a try before asking.

since you post this in plugin development i will suspect that you want to make a kit plugin WITH time feature in state of setting cooldown into someone's kit plugin
so there two routes:
use a callback timer aka heartbeat task
or use time() and compare them

most prefer the easy route look here: (heartbeat task)
https://github.com/ThunderDoesPlugins/DeathBanV1/blob/master/src/Thunder33345/DeathBan/Main.php#L120

PHP:

    private function update()
    {
        foreach ($this->bans as $index => $ban) {
            if ($ban['time'] < 0) {
                unset($this->bans[$index]);
            } else {
                $this->bans[$index]['time'] = $ban['time'] - 1;
            }
        }
    }


(yeah i know it is not the best pratice)
if you wonder what i mean by that
it does work but it create unnessary strains on server for having a heartbeat task running every so often and sorting a bunch of arrays
(in most cases you dont need heartbeat task i could have just use time() and check it with current time on login event)

the second one would be to use time
ex on kit use

PHP:

$this->cabinet[$name]->setTime(time()+$kitCooldownTime)
and to check would be to
if(time() > $this->cabinet[$name]->getTime()){
//means the cooldown have been over
} else {
$time = $this->cabinet[$name]->getTime() -time();
echo "cooldown over in :".$time;
//not over
}


about the $this->cabinet[$name] it was just my way of doing things without confusing arrays littered everywhere you can just use $this->data[$name]['time']
it is just the matter of personal preferences
it was consist of a class with array object implemented if you wish to take a look go here https://github.com/ThunderDoesPlugins/WorldEdit/tree/master/src/Thunder33345/WorldEdit/Utilities
EDIT: changed the code tag to PHP

It is very questionable to use time() in Minecraft at all. MCPC MCPE and MCPC Official server software nearly never use it. I'd almost always recommend using the ticks. Of course ticks can be inaccurate if the server is under high load, but as everything is slower than that is normally what you want.
There are some scenarios where you would of course use time():
- TimeBanning (server could be stopped too)
- Server Uptime (for whatever reason)
- Player Online time

Main reason to not to use ticks are because they are not reliable and rely on server runtime. To be more accurate I always use microtime function

well using time or heartbeat task is alwayls a case by case basis
if you are just making a kit plugin it may just be best to use time
if you are making something that is more dynamic [Insert example] it would actually be better to use heartbeat task

Sorry forgot to put in the code

PHP:

<?php

namespace RoyalMCPE;

use pocketmine\plugin\PluginBase;

use pocketmine\event\Listener;
use pocketmine\event\player\PlayerInteractEvent;

use pocketmine\utils\TextFormat as c;

use pocketmine\command\{CommandSender, Command};

use pocketmine\Player;
use pocketmine\Server;

use pocketmine\item\Item;
use pocketmine\item\enchantment\Enchantment;

use pocketmine\entity\Effect;

class Main extends PluginBase implements Listener{

  public function onEnable(){
    $this->getServer()->getPluginManager()->registerEvents($this, $this);
  }
 
  public function onCommand(CommandSender $sender, Command $cmd, $label, array $args){
    $prefix = c::BOLD.c::DARK_GRAY."[".c::DARK_PURPLE."MCA".c::DARK_GRAY."] ".c::DARK_AQUA;
    $inv = $sender->getInventory();
    if(strtolower($cmd->getName()) == "kit"){
      $sender->sendMessage($prefix."Select a kit");
      $inv->addItem(Item::get(Item::WOODEN_SWORD,0,1)->setCustomName(c::GRAY."Starter"));
    }
  }
 
  public function onSelect(PlayerInteractEvent $e){
    $p = $e->getPlayer();
    $i = $e->getItem();
    $inv = $p->getInventory();
    $prefix = c::BOLD.c::DARK_GRAY."[".c::DARK_PURPLE."MCA".c::DARK_GRAY."] ".c::DARK_AQUA;
    if($inv->getItemInHand()->getId() === 268 && $i->hasCustomName(c::GRAY."Starter")){
      $inv->removeItem($i);
      $prot = Enchantment::getEnchantment(0)->setLevel(1);
      $sharp = Enchantment::getEnchantment(9);
      $p->addEffect(Effect::getEffect(5)->setDuration(1200)->setAmplifier(2));
      $inv->addItem(Item::get(Item::LEATHER_CAP,0,1)->setCustomName(c::GRAY."Starter Helemt")->addEnchantment($prot));
      $inv->addItem(Item::get(Item::LEATHER_TUNIC,0,1)->setCustomName(c::GRAY."Starter Chestplate")->addEnchantment($prot));
      $inv->addItem(Item::get(Item::LEATHER_PANTS,0,1)->setCustomName(c::GRAY."Starter Leggings")->addEnchantment($prot));
      $inv->addItem(Item::get(Item::LEATHER_BOOTS,0,1)->setCustomName (c::GRAY."Starter Boots")->addEnchantment($prot));
      $inv->addItem(Item::get(Item::STONE_SWORD,0,1)->setCustomName(c::GRAY."Starter Sword")->addEnchantment($sharp));
      $inv->addItem(Item::get(Item::STONE_PICKAXE,0,1)->setCustomName(c::GRAY."Starter Pickaxe"));
      $inv->addItem(Item::get(Item::STONE_SHOVEL,0,1)->setCustomName(c::GRAY."Starter Shovel"));
      $inv->addItem(Item::get(Item::STONE_AXE,0,1)->setCustomName(c::GRAY."Starter Axe")->addEnchantment($sharp));
      $inv->addItem(Item::get(Item::STEAK,0,64));
      $inv->addItem(Item::get(Item::GOLDEN_APPLE,0,3)->setCustomName(c::RED."Med Kit"));
      $p->sendPopup("You have recived the starter kit");
    }
  }
}

i will give you idea on how to implement it but i doubt it that i will write you the code to do so for you to just copy and paste it
but i am happy to teach you on how to implement it