php-cli - Controlling external programs

I'm writing a script to be run from the command line. It controls my DAT autoloader (tape changer) and runs tar so that I can automate my backups. With the -M option, tar will prompt for a new tape when the current tape is full. How can I get the signal from tar that the tape needs to be changed? Is there another way to interact with external programs other than, exec, backticks, etc.? I'm running 4.3.2-cli on FreeBSD with POSIX and pcntl functions enabled. Thanks, Mike