If you echo your query, you will notice this issue. Following is your final query
INSERT INTO Users (Email, Displayname, Registered, Rank,Status, Password, Salt)
VALUES ('', '', '{date('Y\/m\/d')}', 9999, 1, '', '')
Notice that your date was not interpolated like you expected it to, and i'm sure if you have that field in MySQL set as a datetime field, it wont accept that value {date('Y\/m\/d')}
, Move the date function call outside the string.
Plus you are not getting any error after the query execution because you are simply not checking for one. One example how to check for that can be
if (!mysqli_query($connect, $addinfo)) {
printf("Error: %s\n", mysqli_error($connect));
}