On a Linux system, running apache2 as a non-privileged user you can not increase the niceness of the process after decreasing it. Also, you can not use the apache_child_ terminate either. I found the following does work though:

<?php

//decrease niceness
proc_nice(19);

//kill child process to "reset" niceness
posix_kill( getmypid(), 28 );

?>

If a process is reniced, then all its children inherit that niceness. So a PHP script can call proc_nice on itself, then invoke system(), and the command executed via system() will also be niced.

Also worth making a note of ionice. There's no PHP function for this, but it's important. A nice'd program will happily try to chew up all i/o bandwidth with very little CPU usage, it can therefore make the entire computer non-responsive despite the programmer's intention.  Use "ionice -c3"  or see "man ionice"

Regarding ionice - on linux the impact of the ionice -c3 class is similar to that of nice, because the CPU "niceness" is taken into account when calculating the io niceness.

It is important to note that this is a relative change. I didn't read the description properly and couldn't figure out why setting proc_nice(0) didn't take the forked children back to 0!
For example if you run:
<?php
proc_nice(-5);
proc_nice(0); // will have no effect
proc_nice(5); // will take the niceness back to 0
 
?>

In PHP CLI under Debian (and probably many other Linux flavours) you can read the 'niceness' from the proc filesystem.  (There may be a PHP command that gives this info but there doesn't seem to be a link to it on this page.)
E.g
<?php
$Current_Niceness_Value = intval(explode(" ",file_get_contents("/proc/".getmypid()."/stat"))[18]);

// Note: Older versions of Linux return an unsigned integer which has to be converted to a signed integer. 
$Current_Niceness_Value = unpack("l",pack("L",intval(explode(" ",file_get_contents("/proc/".getmypid()."/stat"))[18])))[1];

?>

Simple function for check process nice, by default returns nice of current process:

<?php

public static function getProcessNice ($pid = null) {
    if (!$pid) {
        $pid = getmypid ();
    }
        
    $res = `ps -p $pid -o "%p %n"`;
        
    preg_match ('/^\s*\w+\s+\w+\s*(\d+)\s+(\d+)/m', $res, $matches);
        
    return array ('pid' => (isset ($matches[1]) ? $matches[1] : null), 'nice' => (isset ($matches[2]) ? $matches[2] : null));
}

?>