One of your questions was about multiple people reading/writing at the same time. Unless you use some sort of file locking mechanisim (like flock), multiple instances can be started by the webserver and corrupt the data file. So, i suggest that before any reading/writing, you use flock to secure the file.
Other than that, you are going to hear alot of use CGI comments, with which i must say i agree.

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"They shall not overcome. Whoever told them that the truth shall set them free was obviously and grossly unfamiliar with federal law."
    -- John Ashcroft