PHP MySQL script shows no error but won't insert row (more details and different code)

PHP MySQL script shows no error but won't insert row (more details and
different code)

Ok. So right now, my line of code for inserting looks like this:
$query=mysqli_query($con,"insert into orders values
(".$user_index.",".$order_date.",".$_POST['item_number'].",".$_POST['price'].",".$_POST['tax'].",'".$_POST['pay_method']."')");
$order_date is a date. $user_index and $_POST['item_number'] are INTs.
$_POST['price'] and $_POST['tax'] are doubles. $_POST['pay_method'] is an
enum. At the moment, I really couldn't care less about SQL-injection. I
just want a working page for now. I'll secure it before it goes live on my
website.
My virtual server throws exceptions whether I put in
"die(mysqli_error($con))" or not. However, in this case, it is definitely
reaching the code that's in the same if statement after the insert
statement. Yet, when I run this code, everything looks fine but nothing
has changed in the database.
I have tried:
insert into orders (user_index,order_date,item_number,...) values (...)
but it doesn't work that way either.
Finally, yes, I have asked a similar question before. But as I am new to
this website, didn't know if there was a way to edit my old question, and
figured this would just be easier since I have more details now and I
could possibly get the same people as before to answer this with their
questions and troubleshooting steps answered; I just went ahead and posted
a new question.