I need to access the joomla user table jos_users
for login checking from external php script [codeignitor].
joomla storing password like this
If you just use md5($password); it'll work, try it. Joomla has a mechanism and it can work with multiple types of passwords (including, as of late, strong passwords). You don't have to worry about the part after the colon. Just use md5($password) and it'll definitely work.
By the way, this'll also work on Joomla 3.x.