writing to file

i have this app where a user fills out a form and after submit it displays the values they entered. i want to save this info to a file after they submit, then allow them to download the file. some other user does the same, fills in the form and allow that person to download the file... and so on. i don't know how i should go about doing this.. rather than keep creating files i would like to recycle the file each time someone uses the system, but that can lead to other problems.. like several users using the app at once.. any suggestion/ideas/tips how i should do this? basically, just write the info from POST to a file and allow them to download the file. obviously i am worried how secure i will be able to make this. -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.338 / Virus Database: 267.10.2/65 - Release Date: 8/7/2005