sql injection for sessions

Question

I used mysql_real_escape_string() to prevent sql injection for the $field variable below. Should I use the same for $_SESSION['user_id']?

I can't imagine someone being able to change a value in the $_SESSION array. Can they?

$query = "SELECT `".mysql_real_escape_string($field)."` FROM `users` WHERE         `id`='".$_SESSION['user_id']."'";

Answer 1

They can't change the $_SESSION array, but your problem totally depends on how you initialized $_SESSION['id']. In a general way, you should always escape values in a SQL query. Don't try to guess whether or not values can be modified from an user input, just escape them.