php get the last row with MySQL

If you want the last 10 then just change ASC to DESC

SELECT * 
FROM 
chat 
WHERE 
(userID=$session AND toID=$friendID) 
OR 
(userID=$friendID AND toID=$session) 
ORDER BY id 
DESC
LIMIT 10

SELECT * FROM chat WHERE (userID=$session AND toID=$friendID) OR (userID=$friendID AND toID=$session) ORDER BY id DESC LIMIT 10