You have a few options when using PHP, but the right one depends on your hosting environment. If you have access to the semaphore (eg. the msg_get_queue function) or shared memory (eg. the shm_get_var function) functions, try them.

Otherwise, you can get away with some shared file trickery. As long as you are careful you can use the flock() function to lock a shared file when reading/writing data. Each script can keep track of offset of the last thing they read in the file in a cookie/session. You can even automatically do file cleanup when it gets too large.

If you expect few writes, and mostly reads, you can skip tracking file offsets and just have each write add another file in a series (possibly named by timestamp).