Player kills and deaths

What code need use for calculate player kills and deaths ?

I'm using :

public function onDeath(PlayerDeathEvent $event){
$entity = $event->getEntity();
$kills = $event->$entity->get($name);

PHP:

public function PlayerDeath(PlayerDeathEvent $event){
            $cause = $event->getEntity();
            $name = $cause->getName();
            $this->death[$name] = $this->death[$name]+1; //Add Death
            if($cause instanceof EntityDamageByEntityEvent){
                if($killers instanceof Player){
                    $killers = $cause->getDamager();
                    $this->kill[$killers->getName] = $this->kill[$killers->getName]+1; //Add Kill
                }
            }
        }


to get kill,

PHP:

public function getKill($name){
    return $this->kill[$name];
}
//Ex, $this->getKill("korado531m7");
#return kill


to get death,

PHP:

public function getDeath($name){
    return $this->death[$name];
}
//Ex, $this->getDeath("korado531m7");
#return death


using global valuable. so it will be deleted kill and death the score when you restarted server or stopped.
if you want to keep, why don't you use saving to the file (Ex: yml) ?

I'm using .yml file,but with suicide server crash....and idk why..

Please send the errors