Execute every N command in parallel in shell_exec() in PHP - php

Question

I'd like to execute N commands in bash in parallel. And then the next N commands after all the commands finish, and then the next N commands …
Because I am not an expert in shell scripting, I have resorted to PHP. But I suspect my code is not doing what I needed optimally:
<?php
// d() is a function like var_dump()
d($d);
$ips = array(
"83.149.70.159:13012" => 8,
"37.48.118.90:13082" => 77,
"83.149.70.159:13082" => 77,);
d($ips);
reset($ips);
$prx = "storm";
$f = array();
foreach ($d as $calln) {
$ip = current($ips);
$ipkey = key($ips);
d($ip, $ipkey);
$comd = choose_comd($prx);
d( $comd);
$f[] = shell_exec($comd);
d($f);
choose_limiting ($prx);
d($GLOBALS['a']);
}
function choose_comd ($prx) {
d($GLOBALS['calln']);
switch ($prx) {
case "storm":
return "cd /Users/jMac-NEW/HoldingForDO/phub/phubalt_pages && curl -x {$GLOBALS['ipkey']} \"https://catalog.loc.gov/vwebv/search?searchArg={$GLOBALS['calln']}&searchCode=CALL%2B&searchType=1&limitTo=none&fromYear=&toYear=&limitTo=LOCA%3Dall&limitTo=PLAC%3Dall&limitTo=TYPE%3Dall&limitTo=LANG%3Dall&recCount=1200\" >trial_{$GLOBALS['calln']}_out.html 2> trial_{$GLOBALS['calln']}_error.txt &";;
// more cases ...
}
function choose_limiting ($prx){
switch ($prx) {
case "":
if (!next($ips)) {
sleep (80);
reset($ips);
}
case "storm":
if (!isset($GLOBALS['a'])) {
echo "if";
$GLOBALS['a'] = 0;
}
elseif ($GLOBALS['a'] == current($GLOBALS['ips'])) {
echo "elseif";
next($GLOBALS['ips']);
sleep(80/count($GLOBALS['ips']) - 7); // 80 is the standard
$GLOBALS['a'] = 0;
}
else {
echo "else";
$GLOBALS['a']++;
}
}
}
function trying () {
$GLOBALS['a']++;
d($GLOBALS['a']);
}
Firstly, I am not sure if running a loop around shell_exec("command… &") will make all the commands run in parallel.
Secondly, the loop runs around all the possible commands, but is made to sleep() with an arbitrary / estimated duration of 70 after every N commands are run with shell_exec(). 70 seconds sleep period may or may not correspond with the completion of all previous N commands that have been executed, but i am just assuming that it will be around there.
May I know if what I have done has fulfilled my aim? If no, why? And what other solution is there?
Actually I do not mind just using bash directly, but the problem is that every iteration of loop is supposed to be fed with a variable $calln from a php array $d populated in earlier parts of the script not shown. If PHP can do what I need, pls stick to PHP.