Simple question that keeps bugging me.
Should I HTML encode user input right away and store the encoded contents in the database, or should I store the raw values and HTML encode when displaying?
Storing encoded data greatly reduces the risk of a developer forgetting to encode the data when it's being displayed. However, storing the encoded data will make datamining somewhat more cumbersome and it will take up a bit more space, even though that's usually a non-issue.
i'd strongly suggest encoding information on the way out. storing raw data in the database is useful if you wish to change the way it's viewed at a certain point. the flow should be something similar to:
sanitize user input -> protect against sql injection -> db -> encode for display
think about a situation where you might want to display the information as an RSS feed instead. having to redo any HTML specific encoding before you re-display seems a bit silly. any development should always follow the "don't trust input" meme, whether that input is from a user or from the database.
The encoding should only only only be done in the display. Without exception.
Output.
With HTML you can't simply check length of a string (& is 1 character, but strlen() will tell you 5), you can easily crop it (it could break entities).
You may need to mix strings from database with strings from another source, or read and write them back. Doing this application-wide without missing any escaping and avoiding double escaping is a nightmare.
PHP tried to do similar thing with magic_quotes and it turned out to be a huge failure. Don't take magic_entities route! :)
Keep in mind that you may need to access the database with something that doesn't understand HTML encoded text (e.g., a reporting tool). I agree that space is a non-issue, but IMHO, putting HTML encoding in the database moves knowledge of your view/front end into the lowest tier in the application, and that is a design mistake.
Doesn't this defeat the purpose of encoding? If a malicious sql script is entered as input, which is then passed to the db it could cause a huge problem.
来源：https://stackoverflow.com/questions/223480/html-encode-user-input-when-storing-or-when-displaying