Atomic file check/opening?

Is there a function in PHP 4 (I'm using 4.0.1, specifically) that checks or the existence of a file, then creates it if it doesn't exist, all in one "atomic" process? In C under Unix, I would use this: fdes = open("path/to/file",O_WRONLY|O_CREAT|O_EXCL,0); I need this to "lock" a directory, so that only one process at a time can move files into or out of the directory. I have checked the documentation for fopen and flock, but they don't seem to have what I am looking for. Thanks for any help! ---Kris Kelley