someone helped me a bit and now it at least adds something in the database.
indeed I didn't make it clear, yes since I made checkboxes I want to put more genres to a movie. and yes you are right, it only shows the last one.
in the end the person who helped me also told me I should use mysqli and I changed the php part a bit to:
<?php
$a=mysqli_connect('localhost', 'root', 'root', 'movies') or die(mysqli_error($a));
if (!$a){
echo mysqli_connect_errno().mysqli_connect_error();
}
$c=$_POST["Name"];
$d=$_POST["img"];
$e=$_POST["trailer"];
$f=$_POST["air"];
$g=$_POST["airdate"];
$h=$_POST["rate"];
$i=$_POST["genre"];
$j=$_POST["description"];
$sql="INSERT INTO movie(name,img,trailer,air,airdate,rate,genre,description)
VALUES('$c','$d','$e','$f','$g','$h','$i','$j')";
mysqli_query($a,$sql) or die(mysqli_error($a));
echo '';
?>
now I only have the problem with the genres, and the errors that distort the page.
managed to do something about adding empty lines whenever I load the page with
if(isset($_POST['sub']))
where sub is the name for the submit button.