php exec() haphazardly working in pthreads - multithreading

Question

I use a for loop to exec() a file 300 times in pthreads. Sometimes the full 300 exec() calls are successful, but most times a few of the exec() fail and I get between 295 or 299 successful executes of the file.
The error code from the exec() always comes back at #127,
I checked file_exists on failures and it always says "file exists", but it also says "file is not executable" on failures. Which is strange because it was executable the other 295 times within the loop.
My pthread version is only 2 months old. I am very confident that I don't have any pthread workers sharing files or writing to same spots. Scratching my head with what else I can do.
class WorkerThreads extends Thread
{
private $workerId;
public function __construct($id)
{
$this->workerId = $id;
}
public function run()
{
$mainfile="/pastlotto_1/".$this->workerId."/preset_pthread.php";
for($x=0; $x<100; $x++)
{
for($o=0; $o<3; $o++)
{
$command="php $mainfile";
$findE=exec($command,$output,$return_var);
if($return_var !== 0){
echo " file not executed at worker id ".$this->workerId."\n";
echo $return_var."\n";
if (file_exists($mainfile)) {
echo "The file $mainfile exists\n";
} else {
echo "The file $mainfile does not exist\n";
}
if(is_executable($mainfile))
{
echo ("file is executable\n");
}
else
{
echo ("file is not executable\n");
}
}
else{
echo "File executed Successfully";
}
}
}
}//END OF RUN
}
for($r=0; $r<1; $r++)
{
$workers = [];
// Initialize and start the threads
foreach (range(0,2) as $j) {
$workers[$j] = new WorkerThreads($j);
$workers[$j]->start();
//echo $i." worker started\n";
}
// Let the threads come back
foreach (range(0,2) as $j) {
$workers[$j]->join();
}
unset($workers);
}

Answer 1

I was able to solve the problem by changing the code that fires up the pthreads from:
for($r=0; $r<1; $r++)
{
$workers = [];
// Initialize and start the threads
foreach (range(0,2) as $j) {
$workers[$j] = new WorkerThreads($j);
$workers[$j]->start();
//echo $i." worker started\n";
}
// Let the threads come back
foreach (range(0,2) as $j) {
$workers[$j]->join();
}
unset($workers);
}
TO REPLACEMENT CODE:
$p = new Pool(3);
// Initialize and start the threads
for($b=0; $b<3; $b++) {
$tasks[$b]= new WorkerThreads($b);
}
// Add tasks to pool queue
foreach ($tasks as $task) {
$p->submit($task);
}
// shutdown will wait for current queue to be completed
$p->shutdown();