Okay.. now Iv'e seen the full query, I know what the problem is.
The basic syntax for an UPDATE query is as follows...
UPDATE (tablename) SET (fieldname) = (value) WHERE (conditions)
In the query you showed in one of the previous messages, you're doing this...
UPDATE (value) SET (value) WHERE (conditions)
Your probelm is that you've put a $ before your table name and done the same with the field name.
Try this...
$query = "UPDATE cust_tablename SET field_str = $a_value WHERE custid = '$custid'";
Hope it helps.
Regards, Stezz.