从表单输入数据到数据库的麻烦

Question

我创建了一个输入数据的表单，然后将其保存到数据库中。我的数据库是使用PHPMyAdmin创建的，主键是自动递增的（所以这已被排除）。我可以输入数据但只能输入一次，之后不会出现任何添加的数据。任何帮助将不胜感激。

数据输入代码：

if(isset($_POST['insert_post'])){
//getting text from the form fields
$product_title = $_POST['product_title'];
$product_cat = $_POST['product_cat'];
$product_brand = $_POST['product_brand'];
$product_price = $_POST['product_price'];
$product_desc = $_POST['product_desc'];
$product_keywords = $_POST['product_keywords'];

//image from table field
$product_image = $_FILES['product_image']['name'];
$product_image_tmp = $_FILES['product_image']['tmp_name'];

move_uploaded_file($product_image_tmp,"product_images/$product_image");

$insert_product = "INSERT INTO 'products' 
(product_cat,product_brand,product_title,product_price,product_desc,product_image,product_keywords) VALUES ('$product_cat','$product_brand',' $product_title','$product_price','$product_desc',' $product_image',' $product_keywords')";

$insert_pro = mysqli_query($con, $insert_product);

if($insert_pro){
 echo "<script>alert('Product has been inserted')</script>";
 echo "<script>window.open('insert_product.php','_self'</script>";

}