Well, first of all it would be very helpful to use a more descriptive headline. But that's another ropic...
Limesurvey uses tokens for login. So the easiest way is to create a list of customers in a certain format (
see manual -> tokens) and import this data as a csv file into the limesurvey tokens table. Let limesurvey create tokens for each of your customer and send invitations to them via limesurvey. They receive an email which contains a link to the survey and a valid token which is attached to the url.
No username/password needed and no additional PHP scripts.