Problem inserting the mysql bank in php [closed]

I'm developing a project for college and I'm having some problems inserting into the my_sql database, my problem lies in that section.

mysqli_query($conn, "INSERT INTO funcionario(
      codigo_funcionario,
      nome,
      cpf,
      data_nascimento,
      telefone,
      estado,
      motivo,
      periodo,
      cidade,
      rua,
      bairro,
      numero,
      complemento,
      cep,
      cnpj_empresa
    ) VALUES (
      '".$this->getCodigoDoFuncionario()."',
      '".$this->getNome()."',
      '".$this->getCPF()."',
      '".$this->getDataNascimento()."',
      '".$this->getTelefone()."',
      '".$this->getEstado()."',
      '".$this->getMotivo()."',
      '".$this->getPeriodo()."',
      '".$this->endereco->getCidade()."',
      '".$this->endereco->getRua()."',
      '".$this->endereco->getBairro()."',
      '".$this->endereco->getNumero()."',
      '".$this->endereco->getComplemento()."',
      '".$this->endereco->getCEP()."',
      ''
    );");

My problem lies the moment I do

'".$this->endereco->getCidade()."'

I do not know if the problem is with my class, or if it is the syntax that is wrong.

I'm having these two errors "Undefined property" and "Fatal error: Call a member function getCity () on null". Could you give me a hand? Thanks!