How to encrypt a specific column in a MySQL table?

Look here for list of possible encryption functions:

http://dev.mysql.com/doc/refman/5.1/en/encryption-functions.html

You can create trigger for update and check there field accessable. Something like that:

CREATE TRIGGER crypt_trg BEFORE UPDATE ON table FOR EACH ROW
BEGIN
  IF new.accessable = 0 THEN
    SET new.msg := ENCRYPT(new.msg, 'key');
  ELSE
    SET new.msg := DECRYPT(new.msg, 'key');
  END IF;
END;

You also can update all existing records in you table with this query:

UPDATE table SET msg = IF(accessable = 0, ENCRYPT(msg, 'key'), DECRYPT(msg, 'key'));

So you can select records for you PHP code:

SELECT msg_id, user_id, time, IF(accessable = 0, DECRYPT(msg, 'key'), msg) msg
FROM table

UPD. Also here was similar question:

MySQL encrypted columns

You can also encrypt the data prior to the query to insert it, so that MySQL doesn't even know it's encrypted, and decrypt it on retrieval in the application. For that, you should store it in a varbinary or blob column though.