Ok, now I understand.
Lets say the "raw" data is what they typed into the textarea, and the "transformed" data is after you have turned what looks like links into html links.
Take the "raw" input from the textarea, escape it properly and then store that in your database table.
Then, do the "transform" only when the data is to be displayed.
That's probably fine if you are not on a busy site.
If you are on a busy site, or want to save some processor cycles -- you have a couple of options -- these are likely the most popular:
a) Store two versions of the data in your table "raw" and "transformed".
When you want to display the data for the public, use the "transformed" data, when the user wants to edit their field, give them back the "raw" version.
Obviously when the users edits the raw data you update the transformed data too.
b) store the raw version in your db, and create a "cached" html file with the transformed data that you serve up from the file system using the include function -- when you detect that the raw version has been changed, overwrite the cached file too.
You likely need to keep a note of the filename of the cached version with you raw version, so that you can look up its address.