How To Fetch User Table Id Value Into Another Table

Hi, I've three tables in my database namely user, preferences and questions. My preferences table is like this..

CREATE TABLE IF NOT EXISTS `preferences` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`questionid` int(11) DEFAULT NULL,
`answers` varchar(100) DEFAULT NULL,
`user_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `questionid` (`questionid`),
KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=2 ;

My questions table is like this

CREATE TABLE IF NOT EXISTS `questions` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`question` varchar(400) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 AUTO_INCREMENT=1 ;

I given foreign keys to preferences table to questionid from questions table id and user_id from user table id. Now i'm inserting answers values into preferences. Can i know how to fetch questionid and user_id into preferences table. My questions are in _form.php file.