Symfony\Component\Process\Process::start PHP Method

    public function start(callable $callback = null)
    {
        if ($this->isRunning()) {
            throw new RuntimeException('Process is already running');
        }
        $this->resetProcessData();
        $this->starttime = $this->lastOutputTime = microtime(true);
        $this->callback = $this->buildCallback($callback);
        $this->hasCallback = null !== $callback;
        $descriptors = $this->getDescriptors();
        $commandline = $this->commandline;
        $envline = '';
        if (null !== $this->env && $this->inheritEnv) {
            if ('\\' === DIRECTORY_SEPARATOR && !empty($this->options['bypass_shell']) && !$this->enhanceWindowsCompatibility) {
                throw new LogicException('The "bypass_shell" option must be false to inherit environment variables while enhanced Windows compatibility is off');
            }
            $env = '\\' === DIRECTORY_SEPARATOR ? '(SET %s)&&' : 'export %s;';
            foreach ($this->env as $k => $v) {
                $envline .= sprintf($env, ProcessUtils::escapeArgument("{$k}={$v}"));
            }
            $env = null;
        } else {
            $env = $this->env;
        }
        if ('\\' === DIRECTORY_SEPARATOR && $this->enhanceWindowsCompatibility) {
            $commandline = 'cmd /V:ON /E:ON /D /C "(' . $envline . $commandline . ')';
            foreach ($this->processPipes->getFiles() as $offset => $filename) {
                $commandline .= ' ' . $offset . '>' . ProcessUtils::escapeArgument($filename);
            }
            $commandline .= '"';
            if (!isset($this->options['bypass_shell'])) {
                $this->options['bypass_shell'] = true;
            }
        } elseif (!$this->useFileHandles && $this->enhanceSigchildCompatibility && $this->isSigchildEnabled()) {
            // last exit code is output on the fourth pipe and caught to work around --enable-sigchild
            $descriptors[3] = array('pipe', 'w');
            // See https://unix.stackexchange.com/questions/71205/background-process-pipe-input
            $commandline = $envline . '{ (' . $this->commandline . ') <&3 3<&- 3>/dev/null & } 3<&0;';
            $commandline .= 'pid=$!; echo $pid >&3; wait $pid; code=$?; echo $code >&3; exit $code';
            // Workaround for the bug, when PTS functionality is enabled.
            // @see : https://bugs.php.net/69442
            $ptsWorkaround = fopen(__FILE__, 'r');
        } elseif ('' !== $envline) {
            $commandline = $envline . $commandline;
        }
        $this->process = proc_open($commandline, $descriptors, $this->processPipes->pipes, $this->cwd, $env, $this->options);
        if (!is_resource($this->process)) {
            throw new RuntimeException('Unable to launch a new process.');
        }
        $this->status = self::STATUS_STARTED;
        if (isset($descriptors[3])) {
            $this->fallbackStatus['pid'] = (int) fgets($this->processPipes->pipes[3]);
        }
        if ($this->tty) {
            return;
        }
        $this->updateStatus(false);
        $this->checkTimeout();
    }

 private static function setUpPhpServer()
 {
     self::$process = new Process('php -S [::1]:8999', __DIR__ . '/../../Resources/Scripts');
     self::$process->setTimeout(1);
     self::$process->start();
     usleep(100000);
 }