I'm making a table in a MySQL database to save some session data, including session_id. What should be the length of the VARCHAR to store the session_id string?
Depends on session.hash_function and session.hash_bits_per_character.
Check out the session_id page for more info.
The higher you set session.hash_bits_per_character the shorter your session_id will become by using more bits per character. The possible values are 4, 5, or 6.
When using sha-1 for hashing (by setting ini_set('session.hash_function', 1) the following session string lengths are produced by the three session.hash_bits_per_character settings:
4 - 40 character string
5 - 32 character string
6 - 27 character string