Displaying first 20 characters of a comment

Hi. I am trying to find out what the best way is to display only the first 20 or so characters of a comment. The comments which are entered separately by users are stored in a MySql database. I am displaying a list of the last 5 entries on the home page and then linking them to the relevant comments and details. However the comments are to long so I want to display only the first portion, kind of like a intro for users to get an idea and then read further. I am not sure if it is best to do it with the select statement or format it with the PHP. Either way I am not sure how. many thanks Kevin