PHP会话变量有多安全?

I have a login script that verifies a username/password against data in a 'user' table. Furthermore, I have a 'roles' table that specifies the access level of a given user. Assuming I am using safe login scripts, are there any security holes in simply performing an additional query, upon successful login, against the 'roles' table to discover the user's authorization level and storing this into a session variable? The idea would then be that on any page with mixed authority, I could simply query the session variable to discover the logged in user's authorization level. I have a login script that verifies a username/