Using ' to access session variables?

Whats the difference between $_SESSION[foo] and $_SESSION['foo'] I have been using the 's but they seem to be unecessary? I run into trouble if I try something like: $query = " select * from table where (test.id = '$_SESSION['foo']') "; but the following works: $query = " select * from table where (test.id = '$_SESSION[foo]') "; I know there is some way to properly escape characters but is there any reason why I shouldn't just omit the 's? Thanks!