Problem solved!

I didn't want to give up entirely on the flexibility of writing my scripts in C, so I thought some more about how to get a CGI script to use PHP without having to spend a lot of time on PHP logic. My first attempt was to see whether I could substitute the extension ".php" for the usual ".cgi" extension. I could, but it made no difference, i.e., PHP didn't intercept the output. I did some searching in /usr/local/lib/php to no avail, but then I brought up the phpinfo page and studied it. Under the Environment heading, I found the variable SCRIPT_FILENAME and passed it a test PHP filename as a command-line argument. Voila! I got a HTTP header and the expected PHP-processed output! Now all I have to do is use popen from the C program that will become my CGI script, and I'll be able to let C handle the GET and/or POST query strings, create the MySQL queries, and pass a more manageable amount of PHP code to access the tables. Using HTTPS should also be more straightforward that way. I knew there had to be a better way!