PHP Manual: Change the priority of any process

bool pcntl_setpriority ( int $priority [, int $pid = getmypid() [, int $process_identifier = PRIO_PROCESS ]] )

pcntl_setpriority sets the priority of pid.

priority: 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.
pid: If not specified, the pid of the current process is used.
process_identifier: One of PRIO_PGRP, PRIO_USER or PRIO_PROCESS.

Returns TRUE on success or FALSE on failure.

pcntl_setpriority