MySQL

Hi all, Please tell me how to send a null string to be inserted in a MySQL database. If I do something like: $string = null; mysql_query("insert ignore into table(field) values($string)"); It gives me an error. In fact, I have multiple fields to insert, and the error just tell me that was an error at ... and prints the variables I want to imput and I don't know what could be the problem. Do I need to addslashes(), or what else? Thank you. Teddy