.post表单数据到php

Im sending the form data as below:

j.post('data_handler.php', {
        flag: 'signUp',
        username: username,
        password: password_register,
        birthDate: birthDate,
        firstname: firstname,
        lastname: lastname,
        email: email
    },

and the data_handler.php get the info:

case 'signUp': //flag

        // Get the user signup information
        $user = $_POST['username'];
        $firstName = $_POST['firstname'];
        $lastName = $_POST['lastname'];
        $birthDate = $_POST['birthDate'].'T23:30:58.000Z';
        $email = $_POST['email'];
        $password = $_POST['password'];

and passes to a function :

ValidateReg($user, $password, $firstName, $lastName, $email, $birthDate);

... and for some reason the data is not passed, all empty/blank; what i am doing wrong, suggestions/help please, thanks!