I’m running a wordpress plugin and continue to receive errors like:

Undefined variable $admin in C:\xampp\htdocs\wordpress\apps\wordpress\htdocs\wp-content\plugins\wordpress-database\vragen_plugin.php on line 791

This was my code:

    // If ur an admin, you have access to the following 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'])) { $comment="Nee";} else { $comment = filter_input(INPUT_POST, 'comment', FILTER_SANITIZE_SPECIAL_CHARS); }
            $uniq_name = substr(md5(uniqid(rand())),0,10);