Date Comparison with PHP [duplicate]

        if ($data_sistema > $_POST['cliente_data']){        
            echo "Aprovado! Data dentro do prazo!";
        }else{ 
            echo "Reprovado! Data fora do prazo!";  
        }

If the date informed by the system that is also calculated through the customer response is greater than the date reported by the customer approves! If this is a minor problem, this way I did not work the dates correctly then in DAY / MONTH / YEAR value