Is warning guaranteed if it returns false?

["petrov.boris.v.mail.ru via php-general" <php-general@xxxxxxxxxxxxx>]

Is there a formal rule in PHP that any built-in function returning false 
to signify a failure must also issue a warning?

Or, in other words, is relying on warnings being converted to exceptions 
by error handler is equivalent to checking return value, thus makes 
checking return value unnecessary?