$Update = "Update credits set Credits = Credits + 1 Where Username = Admin";
Normally when i do this i keep everything seperated.
Trying getting the new credits value out the side the box
is admin a variable, if it is then you are inserting the variables the wrong way.
$new_credit=$credits+1;
$update="update credits set credits='$new_credit' where username='$admin' ";
Notice how insert the variable in the sql syntax.
tell me if this helps