horafinal := strtotime('08:00');
horaInicial := strtotime('17:00');

horafinal := horafinal - horafinal; //00:00
horaInicial := horaInicial - horafinal; //09:00

diferenca := horafinal - horaIncial; // 09:00

horafinal := strtotime('08:00');
horaInicial := strtotime('17:00');

horafinal := strtotime('23:59') + strtotime('00:01'); //00:00
horaInicial := horaInicial - horafinal; //09:00

diferenca := horafinal - horaIncial; // 15:00