You are going to have to find yourself a PHP rtf parser class that can parse out the control codes and render the text as HTML. There are a few on the net that you can try but keep in mind that the RTF specification is being updated all the time with new control codes. So if the parser class does not ignore codes it doesn't understand it may crash. Good parsers will ignore control codes it doesn't understand and it will keep a look out for \bin or binary control codes when counting curly braces during parsing.
If it was me I wouldn't have had this field in RTF format to begin with. Databases should try and keep styling information out of its content. Styling should be controlled by stylesheets or a program rendering the data itself.
The following link does have a parser as part of the "accepted solution". Keep in mind though that this page will put limits on how often you can view the answer, so once you see it, copy and paste it to notepad and save it immediately. You will need to scroll down to the second half of the page to accepted solution.
Enjoy!
Expert Exchange RTF Parser class as accepted solution