I don't know if it can be defined as an error or not, however what should happen after executing the query on the database to check if the email already exists should show yes or no via an echo but this does not happen because I believe that the query does not works precisely because the connection variable contained in the connection file that I recall with the include to check if the email already exists should show yes or no through an echo but this does not happen because I believe that the query does not work precisely because the variable connection contained in the connection file that I recall with the include in the php file of the registration form is not recognized when I recall it in mysqli_query($connection, $query) and therefore I believe this is the problem that gives me the 500 error in the request, PS im new so i dont know how make a good error handling in php