You can connect a form with the server using any of many technologies, such as PHP, BlazeDS, ColdFusion, JSP, ASP, Perl, Python, etc.
Here are some links for using Flex with PHP and MySQL:
If this post answers your question or helps, please mark it as such.
Greg Lafrance - Flex 2 and 3 ACE certified
Flex / AIR Development, Training, and Support Services