As seen in: https://craftcms.stackexchange.com/questions/24081/plugin-logging-in-craft-3

log-to-file-craftcms.php

<?php

namespace yournamespace\yourplugin;

use Craft;

class MyClass
{
    /**
     * Basic Logging implementation
     */
    protected function log($message): void
    {
        $logFile = Craft::getAlias('@storage/logs/logname.log');
        $log = date('Y-m-d H:i:s').' '.serialize($message)."\n";
        \craft\helpers\FileHelper::writeToFile($logFile, $log, ['append' => true]);
    }

    // usage
    self::log([$varToLog]);