Unexpected column in the database

I’m creating dynamic database from form values but when I run it, it adds extra empty column name as form_id.

            switch ($_POST["form_id"]) { #replaced column names in database cus i'm creating 2 dif database
                case 'kitap':
                    if($sutun_degeri == $form_isim){
                        $sutun_adi = 'kitap_isim'; 
                    }elseif($sutun_degeri == $form_konu){
                        $sutun_adi = 'kitap_konu'; 
                    }
                    break;
                case 'film':
                    if($sutun_degeri == $form_isim){
                        $sutun_adi = 'film_isim'; 
                    }elseif($sutun_degeri == $form_konu){
                        $sutun_adi = 'film_konu'; 
                    }
                    break;
                default:
                    break;
            }
            $sql .= "$sutun_adi VARCHAR($uzunluk), ";

That code block on the top is inside of this foreach (if it will give a tip).

foreach ($form_veri as $sutun_adi => $sutun_degeri) {
            #$sql .= "$sutun_adi VARCHAR(255), ";
            $uzunluk = 0;
            if ($sutun_degeri === $form_isim) {
                $uzunluk = 100;
            }
            elseif($sutun_degeri === $form_konu){
                $uzunluk = 255;
            }

I couldnt remove the empty form_id column so I tried to customize it as Primary Key but it took form_id variable’s value as column name and made a new column.

$sql .= $form_tipi . " INT AUTO_INCREMENT PRIMARY KEY, ";

This empty column is coming from there. I created this hidden input because I have 2 different forms that creates 2 different database tables. It makes separation between forms.

<form method="post" action="index.php">
   <input type="hidden" name="form_id" value="kitap">