pcntl_setpriority() sets the priority of pid to priority. If pid is not specified, the pid of the current process is used.
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.
process_identifier is one of PRIO_PGRP, PRIO_USER or PRIO_PROCESS.
如果成功则返回 TRUE，失败则返回 FALSE。