Playing around with concurrency using Symfony's process component

concurrency.php

<?php

use Symfony\Component\Process\Process;

// The total sleep time of this queue is 22, but it only takes ~10 seconds to
// run as new commands are started as soon as another finishes and the
// $concurrency limit is not met.
$command_queue = [
  'sleep 1',
  'sleep 1',
  'sleep 1',
  'sleep 10',
  'sleep 1',
  'echo "This is an error!" >&2',
  'sleep 1',
  'sleep 1',
  'sleep 1',
  'sleep 1',
  'sleep 1',
  'sleep 1',
  'sleep 1',
  'sleep 1',
];

$concurrency = 5;

$current_processes = [];
$collected_errors = [];
while ($command_queue || $current_processes) {
  $current_processes = array_filter($current_processes, function (Process $current_process) use (&$collected_errors) {
    $is_running = $current_process->isRunning();
    $command = $current_process->getCommandLine();
    if (!$is_running) {
      $error_output = $current_process->getErrorOutput();
      if ($error_output) {
        $collected_errors[] = "Error when running \"{$command}\":\n    $error_output";
      }
    }
    return $is_running;
  });
  if ($command_queue && count($current_processes) < $concurrency) {
    $command = array_shift($command_queue);
    $process = new Process($command);
    $process->start();
    $current_processes[] = $process;
  }
}

foreach ($collected_errors as $error) {
  echo "[ERROR] $error\n";
}