plugins – Warning: Undefined variable $admin

I am working a wordpress plugin and proceed to obtain errors like:

Undefined variable $admin in C:xampphtdocswordpressappswordpresshtdocswp-contentpluginswordpress-databasevragen_plugin.php on line 791

This was my code:

    // If ur an admin, you may have entry to the next web page
    if($admin == '1'){
// When going to /Stel-vraag?createquestion=...
        if (isset($_POST['createquestion'])) {
            $short_question = filter_input(INPUT_POST, 'short_question', FILTER_SANITIZE_SPECIAL_CHARS);
            $long_question = filter_input(INPUT_POST, 'long_question', FILTER_SANITIZE_SPECIAL_CHARS);
            if (!isset($_POST['comment'])) { $remark="Nee";} else { $remark = filter_input(INPUT_POST, 'remark', FILTER_SANITIZE_SPECIAL_CHARS); }
            $uniq_name = substr(md5(uniqid(rand())),0,10);