When I have a webpage that need to do a long calculation and I don't want the server to timeout the process I use 'fork()' to make a new thread to do the work for me. Eg.

# first part of script

# some code

if(fork){
     output("I'm doing some work, please wait.");
     exit;
     }
close(STDIN);
close(STDERR);

# do the rest of the work here
[download]

The above code hasn't been tested but is an example of forking off a new thread.

-caedes