Most likely it's a problem with file permissions. The webserver typically runs with a userid of 'nobody', and is not permitted to write most places. If you're trying to write a file to the user's home directory, for example, it's highly unlikely (and would be highly improper) if the webserver were allowed to write there.

Often, people create a directory off of /home/httpd/, and give 'nobody' special permission to write there.

--Chris

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