There is has a prototyping tool at fusebox.org
It is aimed to ColdFusion but there is a FREE version for PHP also , great!
PHP code generator (RAD tool) , visual modeling and all what it neeed for
serious software dev/collaboration.
Lot of tools for free !! (for PHP is Free all things will be free).
ColdFusion obvious is not free.
Version of Fusebox.org are also for ASP and J2EE
Ideal for people wireframing themselves !