Pigtail - Farm out work/background processes to other machines using PHP
Pigtail was born from a need to farm out processes to other machines in a all-PHP (and often only-PHP) environment.
exec() / pctnl_fork() / shell_exec() / etc. - If all you want to do is run a single process in the background, these are a good solution. Once you decide you want to farm out processes to multiple "worker" machines, this doesn't work anymore.
Gearman - Gearman is a great piece of software, and the closest relative to Pigtail. Unfortunately, as our software often gets deployed on other people's machines (shared servers, VPSs, etc.), we don't always have the luxury of being able to install the Gearman C executable. We needed a pure-PHP solution.