slashes and quotes

Hi, I was looking through some PHP books and scripts on the web and I have noticed different syntax for inputting and outputting data. For example the book PHP essentials used a format of slash and quotations: $sql = "INSERT INTO USER_TRACK VALUES (\"$user_id\", "$sel echo "$user_id"; Another script which was used for discussion group, , used the following format: $sql = "INSERT INTO discussion_board ('$name', '$subject' echo $name; I was wondering what is the difference between using single quotes and double quotes with slashes and the difference between using the "echo" command with or without the quotation around the variable? Thanks. Peter