Passed Data from Ajax Call Not Getting Correctly by $_POST

Question

The first issue I encountered, I have array data that needs to be sent together with string data in one ajax post. I was able to solve it with this:

data: {name_dependents:name_dependent_array, age_dependents:age_dependent_array, dob_dependents:dob_dependent_array, dataString:dataString},

But then I can't save the data I have for dataString through simple $_POST.

Now I'm trying to convert my string post data to array so I can have uniform data (Since the first 3 data are arrays and the last one is a string).

But when I do, I always get this error(Link: my_home_url/POST):

404: Page not found 
This error is generated when there was no web page with the name you specified at the web site.

Troubleshooting suggestions:

Ensure the page you are linking to exists in the correct folder.

Check your file name for case sensitivity . Index.htm is not the same as index.htm!

Temporarily disable any rewrite rules by renaming your .htaccess file if it exists.

Here's the Ajax Passing the data:

function submitClick() {
    var firstname = $("#firstname").val();
    var lastname = $("#lastname").val();
    var username = $("#username").val();
    var mobile_no = $("#mobile_number").val();
    var password = $("#password").val();
    var birth_month = $("#month").val();
    var birth_day = $("#day").val();
    var birth_year = $("#year").val();
    var email = $("#email").val();
    var sec_question_uuid = $("#security_question").val();
    var sec_answer = $("#security_answer").val();
    var civil_status = $("#status_civil").val();
    var gender = $("#gender").val();
    // var a = {
    //     firstname : firstname,
    //     lastname : lastname,
    //     username : username,
    //     mobile_no : mobile_no,
    //     password : password,
    //     birth_month : birth_month,
    //     birth_day : birth_day,
    //     birth_year : birth_year,
    //     email : email,
    //     sec_question_uuid: sec_question_uuid,
    //     sec_answer : sec_answer,
    //     civil_status : civil_status,
    //     gender : gender,
    //     command : command
    // };
    // alert(a); return false;
    var dataString = 'firstname='+ firstname + '&lastname='+ lastname +'&username='+ username + '&mobile_number=' + mobile_no + '&password=' + password + '&birth_month=' + birth_month + '&birth_day=' + birth_day + '&birth_year=' + birth_year + '&email='+ email + '&sec_question_uuid=' + sec_question_uuid + '&sec_answer=' + sec_answer + '&civil_status=' + civil_status + '&gender='+ gender + '&command=signup';
    var name_dependent_array = $('input[name="name_dependent[]"]').map(function(){return $(this).val();}).get();
    var age_dependent_array = $('input[name="age_dependent[]"]').map(function(){return $(this).val();}).get();
    var dob_dependent_array = $('input[name="dob_dependent[]"]').map(function(){return $(this).val();}).get();

    if(firstname == ''|| lastname == '' || email == '' || gender == '' ||password == ''){
        alert("Please Fill All Fields"); return false;
    } else {
        $.ajax({
        type: 'POST',
        url: 'functions.php',
        data: {name_dependents:name_dependent_array, age_dependents:age_dependent_array, dob_dependents:dob_dependent_array, dataString:dataString},
        success: function() {
            $("#message").html('<div class="alert alert-success alert-dismissible" role="alert"><button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">&times;</span></button>Your data has been successfully saved. You can now log on to the site by clicking the Login tab.</div>');
            // $('[name="firstname"]').val('');
            // $('[name="lastname"]').val('');
            // $('[name="email"]').val('');
            // $('[name="password"]').val('');
            // $('[name="verify"]').val('');
            window.setTimeout(function() {
                $(".alert").fadeTo(1500, 0).slideUp(500, function(){
                    $(this).remove(); 
                });
            }, 5000);
        }
      });
    return false;
    }
}

Notice the lines commented out, that's the data I'm converting to array so they are uniform but it's not working so I went back to the previous data I made:

var dataString = 'firstname='+ firstname + '&lastname='+ lastname +'&username='+ username + '&mobile_number=' + mobile_no + '&password=' + password + '&birth_month=' + birth_month + '&birth_day=' + birth_day + '&birth_year=' + birth_year + '&email='+ email + '&sec_question_uuid=' + sec_question_uuid + '&sec_answer=' + sec_answer + '&civil_status=' + civil_status + '&gender='+ gender + '&command=signup';

This is the data when I do print_r($_POST):

Array ( 
[name_dependents] => 
    Array ( 
        [0] => asd 
        [1] => hfg 
    ) 
[age_dependents] => 
    Array ( 
        [0] => 6 
        [1] => 6 
    ) 
[dob_dependents] => 
    Array ( 
    [0] => 2010-07-25 
    [1] => 2010-07-02 
) 
[dataString] => firstname=a&lastname=a&username=a&mobile_number=09176229999&password=a&birth_month=01&birth_day=1&birth_year=1937&email=a@gmail.com&sec_question_uuid=1a78e916f8a2affa1d1de00be7e41f91&sec_answer=a&civil_status=32899cec496b4b25c43c8c4444f24403-married&gender=male&command=signup 

)

How do I fix dataString so I can save the data in MySQL similar to this:

$firstname = $_POST['firstname'];
$lastname = $_POST['lastname'];
$username = $_POST['username'];
$mobile_number = $_POST['mobile_number'];
$password = $_POST['password'];
$birth_month = $_POST['birth_month'];
$birth_day = $_POST['birth_day'];
$birth_year = $_POST['birth_year'];
$email = $_POST['email'];
$sec_question_uuid = $_POST['sec_question_uuid'];
$sec_answer = $_POST['sec_answer'];
$civil_status = explode("-", $_POST['civil_status']);
$civil_status_uuid = $civil_status[1];
$gender = $_POST['gender'];