There are a lot of possibilities, not least of which is that the web user may not have permission to write to the comments field. Check your permissions and check that the objects all have the correct names. It is very easy to rename fields in fmp but your web app developer will probably have to hard-code names in PHP.
Malcom is correct. Field names have to be coded into the PHP script exactly as they appear in the database. If they don't match, they won't work. From your description above, I'd look there first.