Weird things happening with mysql

I have a $query for mysql that looks something like: $query = "SELECT * FROM table WHERE value = 'foo'"; If I mysql_query with foo typed in it works like a charm. If however I try to: $query = "SELECT * FROM table WHERE value = '" . $_POST['foo'] . "'"; $result = mysql_query($query); The query succeeds but returns an empty $result. (I know there is an entry in the table with foo (Primary Key). Anybody have any ideas why this might be? Thanks, -Dan