Rule source on GitHub

DowngradeProcOpenArrayCommandArgRector

Change array command argument on proc_open to implode spaced string

-return proc_open($command, $descriptorspec, $pipes, null, null, ['suppress_errors' => true]);
+return proc_open(is_array($command) ? implode(' ', array_map('escapeshellarg', $command)) : $command, $descriptorspec, $pipes, null, null, ['suppress_errors' => true]);

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp74\Rector\FuncCall\DowngradeProcOpenArrayCommandArgRector;

return RectorConfig::configure()
    ->withRules([
        DowngradeProcOpenArrayCommandArgRector::class,
    ]);