From the website:

What is PHP Surveyor?

PHP Surveyor is a set of PHP scripts that interact with MySQL to develop 
surveys, publish surveys and collect responses to surveys. Once a survey 
has been created it can be published as an online survey (displayed as 
single questions, group by group or all in one page) or you can use a 
dataentry system for administration of paper-based versions of the survey. 
PHP Surveyor can produced 'branching' surveys (set conditions on whether 
individual questions will display), can vary the look and feel of your 
survey through a templating system, and can provide basic statistical 
analysis of your survey results.

PHP Surveyor includes the capacity to generate individualised 'tokens', so 
if you have a list of people you want to invite to participate in a survey 
you can issue each one with a token, and they will be able to access the 
survey using that token. This allows for quite good quality control of 
your surveys.

WWW: http://www.limesurvey.org/