nohup: run PHP process in background

The PHP CLI environment can be different from the web environment (mod_php, FCGI, whatever). It's entirely possible for a script to die with an error when run from the commandline and not when you invoke it through a webserver. Debug your script. If you can, SSH into your server, su to the webserver user and run the script from the commandline yourself.

If you can't do that, set up your own development server where you can do this (it's not that hard if you know some Linux).