I would suggest adding a timestamp column to your table that gets set to the current time when a row is added.

Then you could query the table and order the results by the new column to get what your after.

If you have to limit the number of rows in the table for some reason, you could delete rows 11-n programmatically.

I have a feeling that's not what you're looking for, but here's some quick pseudo-code anyways...

SQL:

CREATE TABLE user_comment (
  comment_id INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  comment_text VARCHAR(100),
  comment_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);

PHP:

$result = mysql_query("SELECT * FROM user_comment ORDER BY comment_date DESC LIMIT 10");

You could also order the result by comment_id descending to get the same effect.