As for the renice function by leandro dot pereira at gmail dot com, this isn't true. pcntl_setpriority() doesn't set the nice level of a process, but instead sets the base priority of it. At first glance this might seem like the same thing, but on a system level, they are actually quite different.
In fact, if you're looking to use pcntl_setpriority() to prioritize your process (a tool or a daemon or what-not), I wouldn't recomend using setpriority at all, but renice it instead. Let the system manage priorities and you'll end up with the results you were looking for.
This applies only to POSIX based systems only (as does the function presented by leandro dot pereira at gmail dot com as well).