Handling file-read failure

I'm kinda new with php, but I'm dying to know how to do the following. But can't seem to get the syntax right. Here's the pseudo code...

Page Refresh = 30 secs
--Read file-a.txt
--file-a.txt not available?
----read file-b.txt
----file-b not available?
------do nothing... (page will refresh in 30 secs and repeat the process)


The problem:
=======================
I have a lot of visitors accessing my site and a lot of times the page becomes inaccessible if it refreshes at just the right (wrong) time while it's being read/written to. Instead of the page locking up or giving a server error, I want to be able to try to read a secondary file as a backup.

How can I achieve the logic above?


Thanks for your help!

Elliott

You can lock files with flock() (http://www.pwp.net/flock).