src/Flash.php
public function has(string $key): bool
    {
        return array_key_exists($key, $this->data) || array_key_exists($key, $this->session);
    }
public function has(string $key): bool
    {
        return array_key_exists($key, $this->data) || array_key_exists($key, $this->session);
    }
Check if a key exists

@param string $key Key
@return boolean