Process locking in PHP

A customer of ours is using a program named microcom called from his own
php scripts to interface with it. He wants to know how he can put an
exlusive lock on the program from php and then release it. He's doing
this so more than one person can use microcom (which is hooked up to a
device sitting on the serial port). He said that flock() in php only
works with file pointers, not processes.