I have an Apache server running on one Pi and also hosts a PHP script that gives the Raspistill command to take a photo. After running through the problems of permissions, I finally got the Pi to take a photo through the web browser. I now want to expand this function, so that every Pi connected on the network takes a photo when I go to the browser to run the PHP script. I am new to networking, so I'm not sure how to go about doing this. Is there a way for me to broadcast this command simultaneously across the network to the specific IP addresses of the Pis? I have heard of UDP broadcasting using sockets but I'm not sure how that will fit in here. Any help is appreciated, thanks.
Apache is an HTTP server, and there's no such thing as HTTP over UDP, so UDP broadcasting won't help. Just run a single Apache instance and write a PHP script which talks to several Raspistill instances over the network.
Some thing like this is worth a try,not too difficult. From the main RPi, get a list of all the units that you expect to run the command/script/process. Configure your system to send a syslog message to all of these. On each of these units, have a local script poll for this message and start a script or whatever you would like it to do.