Description
bool 
pcntl_setpriority ( int priority [, int pid [, int process_identifier]] )
     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。