pcntl_setpriority() sets the priority of
priority is generally a value in the range
-20 to 20. The default priority
is 0 while a lower numerical value causes more
favorable scheduling. Because priority levels can differ between
system types and kernel versions, please see your system's setpriority(2)
man page for specific details.
If not specified, the pid of the current process is used.
One of PRIO_PGRP, PRIO_USER or PRIO_PROCESS.