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.
如果成功则返回 TRUE，失败则返回 FALSE。