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Early in the installation of PHP, I made the naive assumption that users would not be uploading files. I didn't realize that webmasters are users of PHP and would need to upload images (files) - Duh!
In php.ini, there is a setting which should be set to "file_uploads = On"
Next problem was trying to find php.ini. Depending on your installation, it could be in /etc/apache (slackware default) or /usr/local/php/lib (if you installed php from source).