I need a sample programmed in php doing the following:

a) setup a simple html form with two fields 
b) send the result with post to a php page
c) write the content of fields named in an area ["first","third","second"] to a file. the filename is yyyymmdd.txt. if such a file exists append if not create file new and write the field in the order of the array with a  and  at the end.
d) if everything is fine goto  if not .
e) everything should run in a single directory.