I already have my php user registration & authentication set up for my website. What I would like to do is simultaneously register participants in the lime survey database using MY registration php code with the information stored on a seperate database. Thereafter, using my website login, automatically log the user in to the lime survey database with the relevant permissions. Do I need LDAP to do this?

Conversely, would it be easier to make my registration & login php code also write directly to the lime survey database? I.E the participant will have to log in twice (once at the website login and once again to log in to lime survey) but won't have to REGISTER twice.

If you are using Apache webserver you could use the webserver authentication and mod_auth_mysql. But it is a long shot and I am not sure how good the URL match could be done.
However the best would be to create your own authentication plugin, or (if you cannot code) ask the LimeSurvey Partners to code it for you.

Yep : look at [strike] www.limesurvey.org/en/extensions/69-authwpbydb [/strike] www.limesurvey.org/en/community-services...nsions/69-authwpbydb

Link is broken (404)

EDIT: Correct link: www.limesurvey.org/en/community-services...nsions/69-authwpbydb