Call to a member function real_escape_string() on boolean

but not sure why its failing only when there is string in where clause ??
I mean. everything works fine when I select on primary key....

It's not failing because there's a string in the WHERE clause.
It's the exact opposite - a connection is required to escape the string, and because the connection has failed before that (or was never created in the first place), the escaping of the said string fails.

You're not getting the error on a select() call because there's nothing to escape in there and thus no attempt is made to use the DB connection. That doesn't mean it works fine; it will still fail when the query is actually executed, which happens after you call get().

Is it something to do with the sessions library ??

No.