in mysql i have a query as ... where num='355'. but the results for 355 are also returned if i have where num='355+(*';

here num is a int field.

i want it to return 0 records in that case. how do i do that?

Interesting... I wouldn't have expected MySQL to work that way. Here are a few options:

Try casting it as an 'integer' if using MySQL 4.0.2:
http://dev.mysql.com/doc/mysql/en/Cast_Functions.html

Use PHP to validate the value before doing the database query:
http://uk.php.net/manual/en/function.is-int.php

Use a MySQL/REGEXP query to validate the value first:
http://dev.mysql.com/doc/mysql/en/Regexp.html