PHP Fatal error during installation
See this issue: https://github.com/craftcms/cms/issues/4355#issuecomment-498829513

reproduced here for completeness:


  A yii variable 'isWindows' is apparently the way to correct this issue, but I haven't been able to find any documentation describing where/how to set this.


From your Craft config/app.php file, you can use this to set the isWindows property of the FileMutex class to true.

return [
    'components' =&gt; [
        'mutex' =&gt; function() {
            $config = craft\helpers\App::mutexConfig();
            $config['isWindows'] = true;
            return Craft::createObject($config);
        },
    ],
];