pocketmine\entity\Entity::getEffect PHP Method

    public function getEffect($effectId)
    {
        return isset($this->effects[$effectId]) ? $this->effects[$effectId] : null;
    }

 protected function addAttackerModifiers(Entity $damager)
 {
     if ($damager->hasEffect(Effect::STRENGTH)) {
         $this->setDamage($this->getDamage(self::MODIFIER_BASE) * 0.3 * ($damager->getEffect(Effect::STRENGTH)->getAmplifier() + 1), self::MODIFIER_STRENGTH);
     }
     if ($damager->hasEffect(Effect::WEAKNESS)) {
         $this->setDamage(-($this->getDamage(self::MODIFIER_BASE) * 0.2 * ($damager->getEffect(Effect::WEAKNESS)->getAmplifier() + 1)), self::MODIFIER_WEAKNESS);
     }
 }